I was going to suggest Matilda but if he's into more story/drama then maybe not (if you like musicals though you should definitely go at some point because it is fabulous). Drama-wise is harder as mostly it is musicals that are the long-running ones so there aren't that many plays that are on now that will still be there in easter but-
Terence Rattigan 'The Winslow Boy' is coming to the Old Vic in March, that's drama not a musical but the Old Vic generally does fabulous productions plot here
One Man Two Guvnors is good, funny, would recommend that. Is having a cast change soon but I'm sure it would still be great.
Helen Mirren is going to be in a play 'the audience' that opens in february I think about the meetings that take place between the Queen and new Prime Ministers. Obviously noone knows much about it yet but I reckon it sounds good. I'm definitely going!
Curious incident of the dog in the night-time which was totally sold out at the National is transferring to the Apollo in March I think so if you've read that that could be a possibility.
War Horse is also fantastic, I would recommend that as well.
Rowan Atkinson is also going to be in a play, about teachers, called Quartermaine's Terms, which I've just booked for actually because I love Mr Bean
I don't think I've really been much help there, but if you want a play that has definitely had great reviews and you know what you would be getting, I would say War Horse is a safe bet, unless you think he would like Matilda, in which case I would go for that.