I stayed at home with DD. Over the years we did:
Various mother and toddler groups, NCT meet up with other Mums and children, music club, we tried an activity session but DD didn't enjoy that (in your case you have a son who might enjoy one), we took DD swimming - some might do lessons, get togethers with friends and their children, playgroup in later years. That all helped socially, helped me realise what her peers were able to do and what I should try and focus on. Good to get out of the house as well.
If we weren't doing one of the above, we'd always go to the park, for a walk or to the shops where I'd talk about what we were buying. Go into town on the bus/maybe go to a cafe. Often popped in the library to look at book and sometimes they had activity sessions.
At home, she enjoyed play equipment outside - trampoline, swing, slide, we had a ball, chalks for drawing. Some children like sandpits. We planted seeds together, watched plants grow. Regularly read, make up stories and encourage him to look at books. Children also learn through play, so things like catch, kicking a football around, putting blocks together, puzzles, looking at books together. DD loved the maths and English activity books around the age of three and that helped a lot..
When she started school, I was told they did a mini assessment as to where they were in the world and what they understood - I was told DD was 3/33 in the class. So being at home didn't hold her back and she was also the youngest in the year.