Well I would ask you to ask the question why here! That probably is not going to lead us anywhere. Maybe the HOWS should be able to help us a bit here.
1) Talk to him upfront
Well again, since he is the boss, I do understand that the stakes are high, but if you don’t stop, there is a great chance that he might consider it as you showing interest and it can keep going further. I think the idea is to convey the message to him without hurting much of his ego. Maybe one of the easiest things to do is to subtly mention that you are in a relationship with someone. It may not be of a great help but might help in some cases. And again, talk to him – a one on one meeting where you tell him that you are feeling a little uncomfortable in the work place and his demeanour is making you feel a little ill at ease. Do tell him that it might be completely your fault but you’d prefer if a certain distance is maintained. Saying that you might be mistaken but would still prefer a layer of rules will send out a strong message to him saying that you cannot be taken for granted and also tell him that the advances are being noticed and you don’t like it, at the same time also doesn’t hurt much of his ego
2) What if he starts asking about your personal life
There are ways people try to hit on you. You can always tell him that you wouldn’t like to mix the personal and professional fronts and would like to keep it that way. It again clearly tells him the boundaries.
3) What if he starts offering you gifts
Well you can tell him that it is not possible for you to accept a gift if it is given only to you and you can only if the same is given to the complete team. Tell him that it kinda makes you feel that you are being given special attention and you find it uncomfortable.
4) You can tell him that it is bordering on a little harassment
Companies take sexual harassment quite seriously and your boss would hate to be in such a situation. Subtly mention to him that you are well aware of the clauses. I am not saying that you should threaten to complain but saying that you are aware of them itself is saying a lot. He will be careful with you.
5) Tell him that his behavior makes you isolated from the team
Tell him that your colleagues wouldn’t have a cordial relationship with you since they avoid the Boss’ favourite. And you can also tell that this is hindering your performance.
I think the bottom line is still showing that you are putting your work ahead and people will be in a position to appreciate that. If they don’t you always have human resources to help you out.