[MyBirdIsEvil]

If he hasn't ingested anything since being medicated, maybe the medication just isn't effective, what kind did you give him?
Not all dewormers have tapeworm medicine in them, and some of the ones that do aren't all that effective against them.

Btw, you must eliminate ALL fleas from him. If you see ANY fleas on him it means he's infested with them, especially if he's a longhaired dog. If he didn't have any/many fleas on him you'd never see any on him. Is he on any kind of flea preventative? If not he should be, otherwise it will be hard to keep him from getting reinfected with tapeworms, though the probable answer is that he ingested something and got reinfected, or they were never completely eliminated in the first place.

Also, has he been to the vet so that you're completely sure they're tapeworms and not another parasite?

If you're using an over the counter dewormer chances are it's not working. You need to get an effective dewormer that the vet recommends and use the dosage the vet decides would be most effective to actually eliminate them.
A lot of over the counter dewormers are also hard on the dog physically, and the stuff the vet gives is usually better.
It's more expensive but if you've already treated your dog 5 times you're spending more than a one time dose would be anyway.
The pill that the vet gave to my dog (which killed multiple parasites) was 5 dollars.