Recently my boyfriend fainted, he did not lose consciousness and was still awake but was sweating & dizzy after,although he was fine 2/3 minutes later, this has caused me great concern.

Although he is only 23 and fit, he is a heavy smoker and is constantly thirsty and sweats a lot. He refuses to see a doctor, please help !

Possibly diabetic. Get him to a doc.

There might be a few posibilities.

Low blood pressure which causes him to black out. Usually caused by stress and not enough sleep. I personaly had the same issue while 18. Same issue after fainted I sweat like cow. If he still insist not to see a doc, make sure he sleep, drink & eat well.

Both are great answers. I was going to mention about stress but it was brought up. The second thought that crossed my mind was diabetic.
Does he drink alcohol allot?
How much does he smoke in a run of a day?
Even still it would be in his best interest to see a doctor ASAP. This needs to be looked at. Please inform us of the out come. It will give other with the same situation to have a better understanding if they fit into this area.