[TeLMgNva]

You know, I honestly think the absolute best thing you can do for him is to make du`a and ask others to do the same (May Allah guide him. Ameen).

We can all give you the strongest proofs in the world to show him, and indeed some of the brothers and sisters have already done so (and these should be useful, ), but the reality is that proofs don't create Iman in a person, and a person who doesn't want to believe in something will never accept it even if he sees it staring him in the face. Only Allah can grant him Iman, and so the best method is to invoke the One and the Only One who can provide for him Iman.

So, I'd recommend trying to figure out exactly what his issue is with Islam, and then going on from there, while making du`a the entire way. Whatever else you can do will depend on his exact issue. I am guessing it probably has to do more with him wanting to do things that Islam prohibits and cognitive dissonance rather than actually doubting any particular tenet of Islam, and so the best thing may be to make sure he keeps his faith, even if he persists in those sins, while gradually removing him from those sinful actions (because those sins will eventually destroy any Iman he has if he never never abandons them).

A sinful believer is better than a non-believer, and so, if my guess is correct, emphasize the importance of belief in Allah and acceptance of His Commands, even if he chooses to disobey them. He should recognize that what he is doing is wrong absolutely, even if he doesn't see why, as the Command of Allah is absolute.