I'd go with it... talk with him about how "it's fun to pretend you're a baby sometimes isn't it. But did you know there's lots of things big boys can do that babies can't? Like riding bikes, eating treats, etc etc. So sometimes we can pretend you're a little baby if you like, it can be fun. I'm glad you're my big boy though"
And when he's acting like a baby I'd go over the top with "gushing" over him like a baby (though do repeat, oh are you pretending you're a baby?! come here my baby!). Hold him, rock him, sing to him, give him kisses all over and stroke his head lol. Then once he's finished, tell him you're glad he's back to your big boy.
It's a phase and it *will* pass!