I think this is pretty common with our kids - hence the exhaustion we all face!
What works for us is a very stocked craft cupboard and copious amounts of plain printer paper. I also bought 100 packs of things like mini pegs and discs etc to use as math manipulatives. We have membership to education.com which although I wouldn't describe it as the best site in the world, it does have an array of activities and work sheets from K-12 in just about every subject.
I now make DD4 tell me what subjects she wants to learn about in advance and she gets a week minimum of each. (she can do what she wants really but I don't provide different things until the next week, she's stuck with her toys otherwise)I told her it was unreasonable for to expect me to have a complete thought out lesson plan with no notice. This way I also get to plan some fun "kid" things like craft and silly food to remind me she is only 4! Such topics this year were birds, space, spiders, princesses, pirates mice etc.
I plan once a quarter and pre reserve the library books, pinterest my crafts, activities and worksheets etc and plan any field trips. I can still chop and change if nec but it does make it all seem less overwhelming and teaches her some limits & respect while also allowing her to follow her interests. The other thing is, the stuff I buy doesn't need to be used for a single subject so it can sit unused until we feel a need for it. I did start out buying specific things, but now that we have several extra curric the expense is too much.
I wouldn't expect this method to work for everyone, but maybe now he's 3.5 he can prob tell you what his level of interest is and what he wants from you - you could prob form your own "curriculum"
