You need to "TRAIN" him... if you plan to marry this man. He must understand that you don't say things like that ... even if well intentioned... it is hurtful. My DH learned along time ago, to understand that there are certain things he should refrain from saying... one of which is on WEIGHT. He has figured out the "THUMPER" motto:

"if you can't say something nice... don't say nothin at all!"