If your 18 year old still lives at home and is still in High School, yes, I think you can and should let them know he is always welcome to join you all for dinner. If she is not living at home.. it is a little tougher to insist..
But you can certainly always tempt them with dinner at a nice restaurant your treat.. Or a family gathering and include the boyfriend EVERYTIME.. He eventually will feel obligated to say yes at some point.. Hee, hee.
Invite him over for a family game night. We used to invite my boyfriends to church or church events. Family events like reunions birthday parties etc..
My mom always encouraged us to spend time at our home. She was a very clever woman.. The less she liked one our boyfriends.. the more often she invited and included them..
I could then see how he treated my mom and my sister. Usually they really respected and liked my mom and so they respected me. If they were not good to my mom or sister I was done with them.