I am struggling with thumb problems.  I have pain & blisters on the outside of my thumb.  I do not throw fast or with high revs. My thumb swells & shrinks a lot. I have 4 thumb plugs and use tape for fit. I must slowly force my thumb into my ball and I use different tricks to get my thumb into the hole including lip balm, handkerchiefs, a separate insert with round hole. I often used one of my inserts for weeks then must switch to another size only to repeat this over again due to pain & blisters.  Help!

I agree with the above. I can bowl 10 games easily with no swelling or blisters on my thumb. My ball fits like a glove.

If the blister and swelling is on the side of your thumb, it could also be something in your release. If you tend to "suitcase" the ball during your release it can cause the same thing, even if the ball is fitted correctly.

If you concentrate on staying behind the ball(inside of on the side) the problem will probably fix itself.

 
You bring up a good point. A "suitcase" release will create friction, usually more on the index finger side of the thumb. Still, the amount of pain, swelling and blistering described suggests something more serious than just release.
 
Staying on the inside of the ball is a foreign concept to many bowlers. If that's the real problem, he'll probably need some coaching help to correct the flaw.

OP, post a vid for us to see

I love it when they disappear after asking for people to take time to provide input. 

I'd also like to know from the OP if he went to his driller about the problem, and if so, what's his response. That's usually the best first step in these situations.

I have several people I'm working with, introducing tape and switch grips.
Can't believe the fight I get about it.
Just have to let them be until they really want to take a chance and see what it takes to get better.
And then commit to it.

The problem is that unless they start bowling 10-20 games a week, where their thumb and fingers start naturally puffing out, they won't understand and accept the need for tape and interchangeable grips. As much as I think I'm prepared, when I go to the big tournaments in Vegas where I'm suddenly bowling 20-30 games over a couple of days, I still sometimes have issues.
 
People have to learn through their own experiences and failures.