I would suggest that you be a little more encouraging to Alvin, and simultaneously a bit more realistic with his parents.
"Alvin wasn't meeting expectations in school last year as a Nth grader, and so even if he'd been ready for game time, which he wasn't as a Nth grader, we couldn't have given it to him. He's doing so much better in terms of his grades and behavior this year, and as long as he keeps that up, it won't be an obstacle. I can't guarantee him any specific amount of playtime, because that is based on his performance in practice." Praise something that Alvin has done well during practice at this point.