Glove and sling. A good adjustable sling can be even better.
Jacket is all too often used to try to disguise poor shooting technique.
Its an AID but good technique is most important and shóuld come first!
If he starts shooting well, a jacket can improve that further.
As a trainer, I dont even use a jacket on trainees. It spoils them too easily too fast. Technique first!
Dunno wether he likes shooting with an eye blend? Its not expensive and some like using it. Be sure to take a light coloured 1.
Dunno wether he's use his ówn rifle too?
Anyway, if they use 1 rifle model only, he cóuld buy some front sight inserts to fiddle with sizes if he wants and if allowed. Not too expensive either, they regularly sell sechand sets at egun.de.