Answer?

I was getting ready to serve dinner last night and Jonah came into the kitchen to ask for a cup of milk (he had eaten dinner earlier and was getting ready for bed). He told me he knows he can't drink it at the table. He said he knows milk is dairy and we can't have it with meat. I said he was right because it wouldn't be kosher. Then he asked why? And I said because Hashem said we have to eat kosher. He said why? I said because we are Jews and Hashem wants us to. Hashem said we should not eat milk and meat together so we don't.
I could tell he was really thinking about something and trying to process the information and something wasn't sitting well with him.
Then he stumped me....... he said, "But Mommy, what will happen?"

And I didn't know what to say. My immediate thought was, "Nothing." There is not going to be a lightning bolt coming down from heaven. He's not going to get sick from it. And he won't understand the reward of Olam Haba yet.
I told him to go ask his father. And I felt really bad about that. I felt like I should have an answer for him. But I was just totally thrown off. I didn't expect my 3 1/2 year old to be asking what will happen if he doesn't keep kosher?!
Jonah is smart and he has a good memory and he's not going to forget it. So I want to bring it up again and give him an answer.
From a Chinuch perspective - what do I tell him? How do I explain it to him on his level? Is there a way? What should I say?