I had to go shopping for my son the other day
That is nice of you. Which son did you shop for?
It's my future son in law I have to take care of.
A relationship with an in-law can be complicated sometimes and making good impressions on them can go a long way. Doing something for your future son-in-law will bridge the gap between your two families.
I don't want to start feeling like he's a freeloader.
Hopefully he returns the favors and shows appreciation for what you're doing. At least you're trying to help now and its up to him to do his part too.