Truck will not crank over!....HELP ME!!

[sam's 72]

My son has a 98 chevy truck. ( I wanted to post here due to getting more hits ) The other night it would not start. Cranked over like a dead battery. So he put a new starter on it. ( Bought from chevy dealer ship ) still would not crank over.( Sounds like a dead battery, just barely rolls over ) The battery is a side mounted post, I checked it out and it looked bad to me. Where the cables hooked up, you could not get the positive side tight. So I went down bought a top mounted battery, and put it in with all new clamps. The battery is showing 13.8 volts. Still the same thing. When you hit the key, it turns over like a dead battery, just barely rolls over. I do not understand what could be wrong here? All the wire going to starter is good, all connection are good. There is a wire that runs over to an electric box under hood that has a lot of fuses and relays in it. There is a starter relay in this box. Do you think this could be the problem. I figured if the relay was bad, it would not even roll over. What do you guys think??????????. Help me out guys
Sam