Re: Well going to bite the bullet
I think a new barrel for the 7mm would be a very affordable upgrade to increase performance and accuracy, but to get out hunting with your son you'll also probably want 2 rifles. Your first thoughts of a new Sendero or weatherby magnum I think are on track. Or even being patient watching the classified adds here for a used 300. Lots of great deals pass through here for real shooters.
Borden Timberline 300 Win Mag - Hart 26" #4 fluted - McMillan A3 Edge - Jewell Trigger - ATACR