I see boredom as a good thing. Boredom means the child gets the chance to exercise his creativity.
Please do not think you have to constantly stimulate your child or be more "productive" in engaging him. When my kids were this age, they love nothing more than digging into drawers, boxes, cupboards etc. These are great learning experiences.
Instead of TV, my kids are outdoors alot. They come with me wherever I go, whether it is to the market, to the nearby store, to the post office or simply to get mail.
Being SAHM means you have many opportunities to get out and about. Your children will learn so much about the world from being just getting on public transport, watching you buy groceries, interacting with strangers etc. We often go to parks, beaches and other spots of nature during weekdays where it’s so peaceful and quiet.
Get quality toys that allow for open-ended learning. Forget about electronic ones. Sit back, relax and watch him play. My kids do lots of fantasy play ie cooking, cleaning, taking the doll out for a stroll etc. They also play puzzles, blocks, lego, some art and craft if I can be bothered.
And oh, read, read, read to your child. This and the suggestions above are good enough to engage him.