1) don't put a goal on it. To me, the "time" or "quantity" goal of it is not the point.
It is him practicing this exercise. It should be fun. Not a matter of "success" or "failure" due to the "time" or quantity of how many he got.
Just do it as many times as he can... per tiredness or just tiring of it. Then needing a break. Or so that he can realize, that there is an improvement, no matter how gradual or miniscule to doing it. Thus, he gets encouraged.
2) how does the therapist think about it? Does she/he want a quantified number of times, that he has to do it? Or per a timed session?
3) Does it really... matter per his condition, "how many times" or "how many rolls" or "how many catches" he made??? If not, then don't put those notions, on his learning "game."
4) Per the eye patch, you can buy, an elastic band (from a sewing store or craft store), a better one, and hand sew it onto your son's eye patch. Those default elastics they have on it, is, usually flimsy.
Or, hand sew onto it, 2... elastic bands. Thereby, it will be more able to stay on his head. Many face/mouth masks, has 2 elastics on it... which is then able to be placed on the head at 2 places.
My kids didn't have "eye" therapy. But my son had speech therapy. I used to practice with him, the many techniques/exercises that the Therapist taught us. But I didn't put quantity or timed restraints on my son's practicing.... we practiced, until he just needed a break or a change to another exercise. And he still, was able to notice, his own improvement. That in itself, "encourages" the child... instead of getting hung-up on "how many" or "how long" they did it for.
I don't know, just my idea.
;)