what differences does he have?
usually, children pick up at nursery some of the differences between themselves and their peers, or between other peers.
dd2 was aware quite early that she found things more difficult (and had additional support as a result) and so we had always discussed what she found difficult, using age appropriate words. (so at first she would describe herself as 'wobbly', now she tells people she has cerebral palsy)
does he have quite marked difficulties with motor skills? does he see ot/ physio etc? it was these things that first marked dd2 out as 'different' in her own mind, because her friends didn't get to play with the lovely ladies she got to see...
are you expecting him to need a differentiated appraoch to the curriculum? is he statemented/ going to be statemented?
in our experience, in yr r and yr 1 there is a lot of effort put into accepting difference - lots of circle time, lots of disussion. it's usually approached in a very matter of fact way.
dd2 got to the 'why, and why me' stage quite early (she's not 7 yet), but she knows that she was very poorly when she was born and her brain was injured. so, effectively, she knows that her cp is caused by brain damage, and it happened when she was born. she also knows that this makes her wobbly, and talk differently, and need glasses. but she also knows that these are just different things about her, like having blonde hair, and liking skiing and dogs.
tbh we haven't ever sat down and discussed cp - it has just come up along the way. i wouldn't set aside a time to tell him, that would set 'it' apart as something huge and important, and serious. he's not even 4.
i would let it come up naturally next time he finds it hard to do something, and say 'that's ok, do you know, we didn't ever think you would be able to do that? aren't you doing well?' and introduce the reason for his difficulties gently. i wouldn't make a big deal out of it at all. plenty of time for that later, when he really wants to know what causes it, what happened to him in particular, etc etc.