Help with ADD Partner

My fiance and I have known each other for 20 years and been dating for 3. He has always been a quiet guy. 

He has told me that he has trouble focusing. He does alot of little projects to help with that.. model tanks, Star Wars stuff, converting VHS to DVD.  

Its always something.  He has said many times... I think I have ADD. 

Its like something is wrong and he can't see to put his finger on it.

Based on what I read up on... I think there is some truth to it.

He goes through ups and downs emotionally. Sometimes he is very attentive to me and then sometimes very neglectful. I mostly feel like a project of his

that he hasn't completed sitting on his shelf.   

In the 3 years he has broken things off with me and then when he is at a better spot has come back to me better. But a few months in... it backslides and things are not the same.

My point in all of of this.. could the undiagnoised ADD be making his emotions out of wack?

Any insight on this would be helpful.