How can I trust when we don't have the same views on things?

My fiance took a trip to Reno with his buddies who I feel are very negative influences on him. He promised he wouldn't do anything to upset me since he took a trip 6 months before to Mexico and ended up partying until 3am on the beach with his brothers and some girls in bikinis. Well I found out from his friend that while they were in Reno he found some hippie guy on the side of the road and asked him if he had drugs. Well I guess they picked this guy up and brought him to Lake Tahoe with them and my fiance talked him into a deal for his friends. He swears he didn't do drugs and his friends backed him up, but it is the fact he put himself in the situation. We are getting married in 4 months, I have a 4 yr old son to worry about and I work my off... I went to college & have a good job and he spent his last 10 yrs partying, but he says he wants a family life with me and my son, but yet he is always sticking up for his friends. I don't know if he can grow up. I've put everything into us... paying to renovate our house, paying for a family vacation, paying for a wedding... and then I find this out. He just said it was a dumb decision and it won't happen again.