not the best

My male cat is turning 8 months shortly. A month ago I notice he had fleas so I got Bob Martin spot on which I bought in Tesco, would not trust eBay where some people on here have purchased theres. I combed the cat until I could no longer find any flea and put the spot on onto the cats nack , it's says not to get it on the fur but it's near impossible as I found out.The cat was sleepy and abit off but was probably just me stressing it out was fine the next day it also went off its food but I've put that down to coming into maturity . I found that it did not work anyway the fleas where still living so I gave him a bath got the Bob Martin spray and used it on him and everywhere in the house after a day I found dead ones on the cat when it was combed was delighted so continued with that from then on but it was like the began to become immune to it because the came back in force so I give him a bath today and seen all the marks on his skin from the fleas and him biting himself so I'm now goin to take him to the vet instead of wasting my time. Safely say we're both Fed up of fleas! thank God my cat didn't get a reaction like others I think il go vet first from now on