Counting would be where I would start. Use buttons, beans, blocks, whatever you have, and count them. When he can do that (and he may be able to already), do some basic addition and subtraction. (“Look, here are 4 buttons.” Then add a button or two and say, “Now how many buttons are there? Yes there are 5 or 6.”) Let him build it too. If he starts getting the hang of that pretty quickly, you can apply it to daily situations and make little word problems out of it. For example, during snack time, ask him to count how many grapes he has on his plate. Then ask him to eat one grape and ask how many grapes are left. Or, if he’s playing with cars, say something like, “You’re driving 2 cars and I’m driving 2 cars. How many cars do we have together?” If you make games out of it, they rarely dislike anything.
Can he recognize and write numbers? If not, maybe work on that. If so, you could introduce him to the different signs (+, -, =)
Geometry is also part of math, so does he know his shapes well? If so, teach him simple concepts like the difference between a square and a rectangle or that a triangle has three sides and three angles.
You could also get out some coins and discuss the worth of each, and then count them. This is a great way to introduce skip counting by 5 and 10.
Those are some of the ideas I have off the top of my head. I’m positive you’ll get more!
Blessings,
Lindsey
P.S. I know you aren’t using a formal curriculum yet, but I was able to use MUS Primer with my 5yos. They loved it!