Consider that any major repairs that may need to be done down the road may very easily cost more than what you paid for the truck. Given that you're going to be pretty abusive to it when you use it, I wouldn't worry about going to a mechanic to see what needs to be done. Like other said, you might get an earful about what the truck might need, not what it needs to keep it going.
All this being said, the KBB value for that truck (in fair condition, mind you) is in the neighborhood of $3500. If he's pricing it that low, you'll seriously want to pick it over with a fine toothed comb. The rust could be worse than what you see or there could be frame damage. If you run the carfax, you'll probably not see any red flags since it's more than likely he's been the one who has taken care of the maintenance for the truck's life.
Here's a craigslist ad from your area (ish) showing what you should be looking for in an early 00's f150:
https://boston.craigslist.org/nwb/cto/5444111844.htmlWhen you go to see the truck, or if you've seen it, it's your call. Like I said, I would not take it to a mechanic because you're not going to use it as a daily driver. To get your value, I would anticipate doing just the minimum to keep it on the road and to shoot for doing those repairs yourself (great opportunity to learn a new skill!). When you drive it with him, drive it hard. If there aren't any creaks, bends, or funny noises from the frame, and if the engine and transmission seem to be working properly, then offer him less than he's asking.