Ah yes, one of the most classic AR situations

Hopefully he has good adoptive parents!

Most people: "Oh no! you will be a baby forever. How tragic!"
Me if I was on his exact situation: "I see this as an absolute win!"

Staying a bab for eternity, seemingly.

I like this because you can look at it in two specific ways --

1: You have to re-learn stuff that would otherwise be second nature, like walking, writing, and maybe talking properly too, depending on whether or not you already could talk at that age. And you don't get to choose whether you go to sleep at a specific time of day or not.

2: You realize you will be young and playful forever, and can do child-like things, like eating from a high chair, wearing diapers, riding a stroller, and play in a sandbox; without anyone making a fool of you for doing these things.

I think I'd focus the most on the positive side of being regressed. It would be cool if I would be regressed to, like, four years old or so. I'd be young enough to still be ''a small child'', but just old enough to understand the bare minimum of the world around me. That would be ideal.