Welcome!
First of all, unless he is unhappy or stressed, there is no need for you to feel badly about his education. Very young children generally learn best through exploration, play, and picking up and putting down a variety of interests. You are already giving him a number of interesting and enjoyable venues for learning in an age-appropriate way. When you take him to the library, museums, zoos, aquaria, parks, community center, etc., observe what he chooses to do, or the books he selects, or the kinds of activities or topics he is drawn to, or likes to talk about before, during, and after. He will give you hints as to directions that are likely to engage his interest, appropriately feed his need to learn, and give him joy. If there are docents or librarians available, you may be able to ask them for some other activities related to his areas of interest, many of which may be free or low cost.
At this age, mine enjoyed reading (some I could have done without, but I still let them take what pleasure they could from their choices--I think I'm not alone in being beyond satiety on Rainbow Fairies), building creations out of reclaimed materials (aka, trash), listening to and creating a wide range of music, making, authoring, and illustrating their own little books (out of plain paper and many markers), and lots of unstructured outdoor play, with and without actual toys or sports equipment.
