Complete newbie

Welcome to the forum! Sorry about the grandson, I know that has been tough. You have a good goal to work towards for him.
I'm a relative newbie too. Just started last year. I started at 56 with bad feet and knees, specifically arthritis in knees. I had a couple of bad falls snow skiing that had me in pain whenever I tried to run. This went on for years and I just wrote off running for myself.
Started cycling which gradually built up the muscles around the knee and helped a lot fast. Got the tri bug, and on the advice of some new friends at the gym that are into it, I decided to give running another shot. I went to a running store and dropped some money on some good Brooks running shoes and arch supports for my flat feet. I read up on the Jeff Galloway run/walk method and started out jogging for 2- 3 minutes and walking for 1-2 minutes. Only started with 1 to 2 miles. This was April of last year. Between the decent shoes and the Galloway method, I was able to build up to running a 5k for my first sprint tri in August. Nothing short of miraculous for me. I'm doing 7-8 miles usually now, but still do run/walk of 4 minutes run, 1 minute walk for training. The knees have held up well with the gradual increase. And I run slooowwww. I'm in it to finish, not for the podium.
The beauty of tri for me is the rotation of swim/bike/run that keeps from over training and injury in any 1 area and allows plenty of rest between each training session for each discipline. Do some research on the Galloway method, maybe get a couple of swim lessons if you're not sure about your stroke technique, and just do something most days.
Just start off slow, but keep at it and you'll be amazed at the results and how fun it is. My favorite tri saying is "it's good that I'm only competing with myself... because everyone else is kicking my a$$ !" Best of luck!