I'm not a grandparent yet, but to speak from my experience as a grandson in my family, I would say I was spoiled just enough by my grandparents and things turned out pretty okay. I would imagine it is hard to keep your emotions from trying to show love by buying gifts frequently, but everything in moderation is okay.
If you do feel that you are giving too much, just consider some alternatives instead. Our grandparents and extended family would sometimes give gifts in the form of chipping in some amount into our college funds. A nice alternative gift idea could also be for an 18th birthday car fund or some amount towards it. I know I would have been pretty lucky to have a nice car when I was 18.
You cal also try offering him to work or do some chores in exchange for the stuff, our family always used this to teach us discipline and at the same time, we were able to get what we wanted.