How to keep a relationship lasting/ Age gap?

I’m 15 years old I’m turning 16 in March, and he’s 17 turning 18 in a month. We’ve been dating for 3 months and he’s a good guy, doesn’t’ do drugs doesn’t pressure me etc. a sweet guy. I know it’s early to ask but I need tips and advice to make the relationship last as long as possible. He’s going to a college nearby and hes determined and willing to drive to be with me. I just kinda want advice to keep it lasting. Sorry if this sounds stupid.