If there is a very strong rapport and attraction between a man and a woman will the man eventually tell her or do/say something that makes it obvious?

Not necessarily, he will need to be available himself, he will need to know you are available and you would need to give him some signs that he is likely to be successful, if he asks you out.  It also depends on whether the guy is used to dealing with women or not.  If he's shy, he'll keep putting it off, persuading himself that she probably won't fancy him.

I feel closer and closer to someone everytime I see him and never expeienced feelings like this before. He has a girlfriend and I'm single,

He's less likely to make a move on you if he has a girlfriend.  

I saw a photo of his girl and she is quite unnattractive and big built.

Women think looks is the most important thing to me, men even think that themselves.  But the truth of the matter is, that personality is just as important.  True, you are not going to get many men if you are really fat and ugly, but the fact is some curvy women are attractive, because they have a brilliant personality and some guys are even more partial to a curvy woman.

I am the opposite of this and look much younger than I am so do I stand a chance?

Yes, you stand an increased chance, paritcularly because you say you also have a rapport with him. 

Is he likely to do something eventually or what?

If he fancies you and things are not working out with his girlfriend, then eventually he will make a move.  But if there's even the remotest chance that he adores his girlfriend, then things are not going to happen with you until he's made a decision that she is definitely not for him and you are a better choice.

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