A toothy grin split Mike's face as he realized what his son had done, his meaty cock already half-hard and drooling into his thick leg fur as he scanned the previous transformations. He had no desire to undo what the boy had done, and in fact seeing what Jeff had wanted only reassured him of Jeff's place in the new familial order. Mike was perfectly happy keeping Jeff as he was--and now that tiny piece of human still left inside him was happy too.
The auto-update which had synced Jeff's DNA to Mike, turning him into an anthro lion as well, had had the dual effect of de-aging him to a more suitable age. As a part-lion Mike's fertility spiked later, meaning that any children would have to be younger. Not that Mike minded; he thoroughly enjoying having his fuzzy twink of a boy around for a while longer. And the pack mentality and his own thick musk would keep his son ass-up long after he'd become a grown lion himself.
Mike moved the mouse over his settings, pleased but not entirely satisfied with what he saw. For once his human self was in agreement with his lion self that he wanted to be better. Stronger. A true pride leader. So it was with that in mind that he began increasing his statistics, filling out his muscle tone, testosterone output, and cock size until he was satisfied that no one could challenge him. Before he had been impressive; now he was intimidating. His joints cracked and popped as he grew taller and taller, his muscles bulging and thickening until his veins could be seen even through his golden fur. His cock grew as fat and long as a man's forearm, silken blond hairs covering its sheath as the head grew fat and red. As his testosterone doubled his body hair grew thicker, the fur on his chest lengthening until its thickness was as distinct from the rest of his body fur as a normal man's chest hair was from his skin. The chair creaked and moaned as his growth came to a rest, his cock brushing against the floor, and his animal musk filled the room, sinking into every article of clothing and making Jeff whine in his sleep.
However, that wasn't quite enough for Mike, and, glancing back at his cum-drenched son, his feline urge to have a pride of his own reared its head. The nature of the transformation made him seek a breeding partner and, having no desire to mate with a bland human female, a truly bestial idea popped into his head. Lions would often mate with their own kin when siring offspring and Jeff, son or not, had been registered in his father's mind as his partner. Had Mike been a bit more human he might've ignored the urge but as he had changed so had his inhibitions, and the pull of his sex drive far outweighed mere blood ties.
A few clicks on the Chronivac and the job was done. Mike stood from the computer, the floor rumbling as he walked over to the bed. At his new size his head brushed the ceiling, his cock over a foot long and as hairy as the rest of him. He let it hang above his son's ass, enjoying the obscene size difference--Jeff's thighs were smaller than his monster of a cock. His son trembled as Mike pressed it against the boy's crack, letting his precum gush out to slick his hole.
Jeff stirred, eyes blinking slowly as he awoke, but before his mind could even register what was happening his instincts took over, spurred on by the musk and the changed Mike had made to him. Instantly a warm wetness began to gush from his asshole, slicking the tip of his father's cock as he let out a whine and arched his ass into the air pleadingly.
Mike didn't even hesitate. He grabbed onto Jeff with his enormous paws--easily encircling his boy's waist--and pressed the head of his cock against his son's pucker. On any normal human it would've split them in half but one of his changes had been granting Jeff an ass built for taking cock, and so instead of ripping it merely stretched to its absolute limit, his cherry-red hole clamping onto the head of Mike's cock like a vise. Jeff cried out in a mix of pain and pleasure but his father gave no mind, too focused on burying himself in the tight heat before him. Jeff's belly bulged grotesquely, the teenager clamped onto the shaft like a glove, and Mike wasted no time in pulling back out to thrust again, a loud slurping noise coming from the hole as it reluctantly released its grasp. Mike pulled out to the tip before slamming in with all his immense strength, spurring a scream of delight from Jeff. He thrust again, then again, the silken fur and gushing precum keeping his son slick and open for his dad's animal dick.
It wasn't long before Mike came, roaring at the top of his lungs as he shoved into Jeff as deep as he could, his white-hot semen flooding the boy's rectum, unable to escape because of Mike's shaft and so backing up and making his stomach bulge further. Mike's giant lion testicles shot for three minutes straight until even his stamina slowed, his body dripping sweat as he looked down at the purring Jeff. His son's belly was distended and Mike slowly withdrew his cock, with it coming a gushing flood of his sperm which spurted all over his hairy legs. He paid it no mind, however, resting one hand on his son's fuzzy abdomen and remembering what he'd typed into the Chronivac.
Right now, inside his son's new womb, his seed was taking root.