Hi there and welcome to the forum!
The only advice I can offer is to put stop on marital plans until fiancé sobers up completely (as in abstaining from alcohol plus having consistent support program) and goes like that for a year. I realize that this is not what you want to hear.
His family is another concern - these are potentially future grandparents, aunts and uncles of your babies. Even if he transforms his life - there is that....
How to bring it up to him? You have to have a firm boundary in place and let him know what behavior is unacceptable to you and always, always follow through
Right now there is a very little incentive for him to quit - he gets to drink (albeit not in your presence) and gets to call himself your fiancé. I have married a sober alcoholic who was working the program - even then chances of success are not that great (I am divorced single mother now).
Good luck and hope you stick around here - this place is a life saver!