S.C.
Start with things like the Gerber Puffs (or the generic version), those dissolve in the mouth, so very little risk of choking. Cheerios are a good starter finger food too. Other than that, you really can give them anything that you're eating (with the exception of nuts, peanut butter, or honey). Cut the pieces up very small so he doesn't choke. I would say probably about 1/2 the size of the tip of your finger. If that's too big, make the pieces smaller.
Yes, you should cook veggies first. It doesn't have to be super mushy, but soft.
Babies can gum just about anything! How many teeth they have is absolutely no indication of what they can or can't eat. My girls were both late teethers and were totally on table foods by the time they were 1.
Once he gets the hang of the finger foods, you can make the pieces bigger. As far as how to serve it, just throw it on the high chair tray. It's also a lot of fun to give them a baby spoon to play with while they eat. At first it's just for banging on the tray and making lots of noise, but they eventually realize it's for eating.
OH! I should also mention that anything with a skin (grapes, apples, hot dogs, sausage links etc) should be peeled before you give it to your son.
Experiment and have fun! It's nice when they learn to feed themselves :)