My cat has these dark brown black spots on his stomach, I searched all around Google and I couldn’t find answers, he currently has treatment for fleas. I’m not sure if fleas are at fault.
Hi and welcome to TCS! Use a flea comb to see. Get a dish of soapy water, ideally with something that doesn't discolor the water too badly. Take long strokes with the comb, and dip it in the water, and wipe it off with a paper towel in between strokes. If there are fleas, flea eggs, and flea 'dirt' the water will likely turn reddish-pink and you may see dirt, and possibly some fleas - which will also happen on the paper towel too.
Many of the flea treatments just aren't as effective as they once were, and there are locations/regions where the fleas have built up an immunity to some of them.