Congratulations on your son’s recovery. He did the right thing by shutting down for 2 months - GOOD FOR HIM!
As he found out, doing gradual demands on his body - any part, does have its rewards. Such work, allows the body to gradually adjust, condition and pace itself. Smart move on your son’s part. And my repeated congratulations to you dad for being there to support him.
With respect to blisters, calluses and such, I’ve had pitchers who had to mix their inventory with a breaking pitch, of sorts. The middle finger was used to pressure point the seams - or - the skins of the ball for movement on the pitch. This development stage in their repertoire took some getting use to - hence an irritation would develop, and thus over time, a callus would develop.
Your son on the other hand, does not have the “hide” texture of a mature man who does this for a living - hence the blistering effect.
I would not suggest a lotion or other ointment. I would suggest letting the blister run its course. Go easy on the use of the finger and the pitch that he’s using that demands so much on that finger.
Repeated use and exposed skin tissue will lead to infections and a higher level of pain, discomfort and a prolonged period of healing. Again, I would avoid ointment’s and such only because of the potential of exposed skin tissue now and/or down the road.
By the way, placing a bandage on the blister and pitching, may cause some issue with a league rule that your son is competing with. He may be called on for a foreign substance on the hand that puts an influence on his pitchers.