I'm getting older and I just want a nice, comfortable, proper relationship. I don't want to be someone's 'good enough for now'.
Most of us who have been a bit older meeting a partner have felt this way. It is completely understandable.
am I your girlfriend or just someone you are happy to have around in the absence of an girlfriend?
This sounds like you undervalue yourself. Please don't say it. If we undervalue ourselves, it leads the way to others undervaluing us too.
Reading what you have written today, I think I would just ask him directly. You can do this without sounding as if you are asking him to commit to you for the next ten or twenty years.
I would urge you to take the lead and tell him how you feel, that you'd like to take the relationship to the next level and call him your boyfriend. Say it jokingly if you like or tell him its your autism - say it in whatever way you feel is best - but ask him if he feels the same way? If he doesn't, then he is not the one for you. If he does feel the same way, it is fair that you treat him as an equal and expose your own feelings before asking him to expose his.
Remember this could be the beginning of a long relationship with this man. It has to be worth a shot.
Have a glass of wine first :) I genuinely hope this works out well for you.