1986 Ford F-150 Won't start

"I am having problems getting my son's truck started. He drove it home last night and it quit running. Thinking it was out of gas he went and got some and put it in but it still wouldn't start. I went over there today to check it out. Checked the spark first and it was getting good spark. Pulled the fuel line off before the filter and checked the flow coming out of the pump and it was flowing fine. Checked the flow after the filter in the fuel rail and that was ok. Went on and replaced the fuel pressure regulator and that still didn't solve my problem. It acts like it wants to start at first but doesn't. I can prime the throttle body with fuel and it will run until the prime runs out. He don't have a lot of money to spend on it and I don't want to do a lot of costly guess work. Help me."