Yes, gel inserts/insoles. I do that for myself sometimes.

Go to a kids' shoe store and buy inserts. It will cut down on the extra space in the shoe.

Go to Target and get some of the Dr. Scholl's gel inserts – they come in heel inserts (and can be quite thick) or full sole size. I did this once with my DD's ballet flats, as one shoe was slipping off her heel when she walked. It should take up enough of the room that it should get him through until he grows into them.

Two pairs of socks

Are they lace up? If so, redo the shoestrings in the top holes (closest to the leg). Instead of having the string under and then up, reverse it so it goes into the hole from the top, goes down, and comes out right by the tongue. I hope this makes sense. This method tightens the shoe a little better around the ankle.

You could probably get an extra sole at any store and make it smaller.

My son's sneaker does not come in 1/2 sizes, so we went up rather than buying just-so fit.

Now the problem is his foot slips by the heel/ankle region.

Any suggestions? Is there and insole or pad created for this problem?