Personally, and obviously everyone is different, I’d be very clear with him that when he is older, and has learnt lots more about Jesus, and has carefully prepared, then he will be able to receive Communion like the adults do. I would also point out the children older than him who are not receiving Communion, and point out how nicely those “bigger boys and girls” are going up for their blessings.
I would actually avoid going too much into “why” he cannot receive right now, apart from saying that he will be able to when he is older and has learnt more. He is learning about negotiation, and he will think it is negotiable if you go too much into what is preventing you from saying “yes”.
If you must, you could even talk about how “when you’ve been going to big school for a few years” and “have learnt lots more about Jesus and what Communion is” and “have got much bigger”, “you’ll receive Communion for the first time, and we’ll have a party to celebrate” and “we’ll invite [insert relatives]”. “Won’t that be exciting? But not until you’ve grown up, and been to big school for a while”. Maybe reinforce again “see how big those children are, and they don’t receive Communion yet”. Perhaps even “it won’t be until we’ve had your birthday, and Christmas, and your birthday again, and Christmas again, [1, 2, 3, 4, 5,…(counting up)] times.”
Basically, I would make it something that happens at a given point that he hasn’t reached yet, and won’t reach any time soon. Make it as certain as starting school, or celebrating Christmas: you can’t do it earlier.