I'd give him $2,500 if it had a plow on it. The way it sits, I'd only pay $1,800. You would have to sink over $1,000 into it just to make it worth close to 3k. 80k mls or not, that rust is cancer. I'm willing to bet that if you took a hammer to all those spots, cab corner in particular, you would probably need to replace more metal than you realize, been there. How are the tires, steering box, and tie rod ends? Those parts add up fast!
Don't forget about the junk yard, I found a Dodge Class III reciever for $40.
Good luck and your old man probably knows best! I know I should of listened to mine a few times.