Hi there!
when my DS was 12Wo i had to change him to a 4hr EASY 'cause he started looking really bored with his bottle, and he wasn't that much hungry, and his naps started to be a mess, so i did it cold turkey, and it all fell back into place, the first week was a bit hard to keep him more at Atime, but i distracted him by walking him around the house (which he loves). So, how much is he eating? how does he behave when he is eating?
What you could also try, is for a longer routine, but include cluster feeding if you think he has just hit the 3MO grow spurt.