Of course, you could purchase supplementary curricula, and just pick up mathematics from his mastery level and follow him forward. That would be the most straightforward. But if you don't want to actually accelerate his instruction, just give him age-appropriate mathematical play, then...
For concepts, rather than pursuing computational fluency, I think the most kid-friendly approaches involve everyday context, like dividing trading cards for a group of friends, determining how many smiley fries you need to put in the oven in order for everyone in the family to have a specified amount, and scaling up or down for cooking. Counting pips on playing cards can also be fun, or figuring out how many shoes there are in the family if each person has a certain number of pairs, etc. Comparison shopping with unit pricing is also a useful and natural activity.
A fun and inexpensive DIY is to take two mirror tiles and tape them together along one edge, securely enough that they won't easily separate, but loosely enough that the tape will act as a hinge. (Obviously, tape around the remaining edges too, and maybe some contact paper across the non-reflective side, to reduce the chances of broken glass incidents.) put an object inside the angle between the two mirrored faces, and experiment with how many images you see as the angle increases/decreases. Put multiple objects inside the angle, and you can now play with multiplying by the number of reflections.
