This was my cab corner last year.
I bought a new one from LMC.
I had a local body shop do some work for me.
They did a few other small repairs for me too. I didn't want to practice my (lack of) welding skills on the outside of the truck, as I am going to paint it back the original color of black. I am going to do the welding on the floorboard myself though. Although, I am not going for show quality, I am wanting to do a good quality job that will last many years to come.
If your son can weld some, then I would get him to help you do the job correctly. And you will have to use some (very little) Bondo as a filler for the imperfections left after the weld gets ground down. I would also use some 2-part epoxy primer to seal everything from rusting again. Good luck with your project.