A suggestion here from someone who gradually introduced my wife to the sissy and then baby side of me without any big shock. Though of course, it is a bit different the other way around.
You could start by asking him to shave his genitals (or let you do it - more fun that way). Once done, look approvingly at the result, tell him how pleased you are, hold his cock in the palm of your hand, say how sweet it is, and how it looks smaller "just like a baby boy's now." Stroke it, and ask whether baby boy would like a rub. Of course, he'll say yes, so give him the slowest, best hand job you can, occasionally (not too often) making comments like "ooh, baby's getting harder", "ooh, does baby like this", and when you finish, ask him if baby liked it. When he says yes, say, "Oh we'll have to do it again tomorrow won't we" (he'll almost certainly say yes please!) and leave it at that.
Reward him for the next week with amazing HJ/BJs, each time asking beforehand if baby wants it. Start to introduce mummy "does baby want his rub from mummy now" and if at any time he pushes back, you can say "oh dear", and nothing happens, so he gets to associate pleasure with him being a baby and you the mummy. Start calling his member something more babyish - mine is a winky (or winklet to emphasise it's tiny).
Once he's conditioned to being called a baby, you need to gently take the next step. It's a good idea to do this first by suggesting it as he is building up to a climax. So for example you might say "Ooh baby's very noisy, I think we're going to have to put a pacifier in you, don't you". Again, if he says no, end the session (kindly but firmly) with him unsatisfied, and without having shown it to him. Next time say it again, and he'll probably remember that saying no isn't a good idea, and say yes. Get him to say "yes please Mummy", and then you have the go ahead and can pop it in him.
Tell him you love him being your baby boy, and it gives you a lot of pleasure. Let him suckle you whilst you stroke his hair. Introduce spoon feeding of porridge (or similar) whilst he is cradled in your arms. Call your sessions "milkings". Make him know that there is no shame in being your baby and he is doing it for you. Eventually tell him that baby needs something pretty to wear in bed, and buy him a relatively plain baby outfit - nothing too ornate at first. I particularly like this style, which was the first one I introduced to my wife. (It comes with frilly panties and a bonnet, although again, all of that might be too much at once.)
So think of it as a journey over several months - too quick and you might scare him off. Make him associate being a baby with his own pleasure, and gently but firmly reward him when he goes along with you, but doesn't get satisfaction if he pushes back. It may take you several months to get to the stage of diapers, and other things, such as having a separate "nursery" (at first occasionally I'd suggest), but eventually you should have completely conditioned him and get him to enjoy the experience of being loved by a mummy. Good luck, and let us know how you get on however you choose to approach the subject.
