I wouldn't spend money on anything at his age, unless he's asking for it. 3.5 is so, so, so, so young. I know it doesn't seem like it when they're your first or only (been there!) but it's true. If he's asking for it that's great, but if he's not, don't stress yourself out even more by trying to add in things that really just aren't beneficial for kids that age. I have an 11mo and an almost 7yo, so I'm at the point of HAVING to do some formal work (we definitely aren't unschoolers) and there's just no way I'd put myself through that for a kid your son's age.
In fact, consider yourself lucky that your daughter will be older by the time your son is ready for K or 1st grade, lol. Today my 11mo learned how to climb onto my 6yo's table. Yeah, that's not very helpful when the 6yo is trying to do math. On a normal day, while we're trying to do school work, the 11mo tears books off the bookshelves, eats paper, pokes herself in the eye with colored pencils she's snagged from the pencil box, plays in the toilet (because the 6yo forgot to close the bathroom door), throws keys or phones in the trash, takes junk mail out of the trash (and eats it), chews on the white board eraser, goes into the 6yo's room and finds small toys she shouldn't have and puts them in her mouth, tries to rip our maps off the walls, gets down the hall and into the dog's water bowl before I even notice she's gone (she's fast!) and all kinds of other things, just in the 90 or so minutes we're doing focused work. Really, truly, it's that crazy. Plus, she's a baby and she likes the sound of her own voice, so she shrieks. A LOT. Try explaining any new concept to one child over the happy, but annoyingly loud, shrieks of the other. Don't put yourself through that before you have to. lol