mama advice

My oldest will be 10 in a couple of months (boy), hitting the preteen years. Any advice from you mamas. My SO has been his main father figure since he was 5, before that it was my dad (my son's bio dad was caught up in drugs & ended up going to prison for a few years, he just recently came back into his kids lives 2 years ago & it's still rare.) Him & my SO are super close, I wasn't very close with my parents & my SO never knew his dad & his mom worked nights so he only had older siblings... enough rambling- my thing is, how do I go about talking to him so he knows he can talk to us about anything & not feel like he has to hide things? he does really well with telling the truth but I know when I was in my preteens/teen years I hid things from my parents, hungout with the wrong crowd, smoked weed at 14, lost my virginity at 15, etc. I just want our close relationship to continue! any mama advice?