Just because your cat is still scratching (as, from your post, I assume that he is) doesn't mean that the treatments aren't doing their job. The basic itching can last for at least a week or two after he's been bitten but, if he's had an allergic reaction to the bites, it can go on for at least a month (and possibly quite a lot longer).
Try using a flea comb. If you don't find any fleas, then the treatment is working and you just need to give it time until the itching stops. It's only if you find any fleas that you need to be considering alternative treatments. (Credelio works well, and it's really easy to hide the tiny tablet in a piece of chicken, but it's not cheap!)