Firstly, his behaviour falls under the umbrella of various mental disorders, based on the symptoms you have described. The only way to be sure is to seek professional assistance and advise. Secondly, I have found that family members, because of the long standing relationships they have with each other, they do not always identify a member's behaviour and associate with a mental disorder. Due to the number of years they spend together and the experiences that they share, they may have just learnt to take the good with bad and chalk up behaviours as 'just his personality'. They have seen him laugh as much as they have seen him cry and all of that just adds to family experience. As a new person and when I say 'new', I mean someone who has not known him his entire life, you may be more likely to notice things that are "off".It is kind of like an 'outside looking in' scenario. Also, because you are his fiance, he might show you more of his 'bad side' or 'bad moods' than he would his family members.It is in the same way one would probably cry to a girlfriend about relationship woes than family members sometimes.
Finally my dear, it is important to acknowledge that you are human and you deserve all the wonders and happiness life can supply. I urge you to consider the life you want to reflect on years from now. This man is grown and irrespective of whatever conditions he has, he does not have the right to treat you poorly simply because he feels like it. You need to stand up for yourself and let him know that you are worthy and he needs to treat you well. He needs to get help and you can support him but you should never place yourself in a position to be anybody's punching bag. You are important too.
Wishing you peace and happiness,