2017 laws 11.7. Sanction is PK.
Indeed 11.7 OFFSIDE AFTER A KNOCK-ON
When a player knocks-on and an offside team-mate next plays the ball, the offside player is
liable to sanction if playing the ball prevented an opponent from gaining an advantage.
Sanction: Penalty kick
The Law makers in the re-write have removed this (I hope no one is going to suggest that the law has changed and this is no longer offside). Why did they remove it. Well I guess it is because Offside (in open play) - Whih this is is already dealt with under Law 10 OFFSIDE AND ONSIDE IN OPEN PLAY.
So Does this law (10) help us at all?
How about 10.5 A player is accidentally offside if the player cannot avoid being touched by the ball or by a team-mate who is carrying the ball. Only if the offending team gains an advantage should play stop. Sanction: Scrum.
I would put the case that if the player could not avoid being touched by the ball. The fact he caught it was only because the ball came at him so quickly he had not time to react except for the "auto" reaction to a ball coming at you at speed from close quarters. The call of scrum seems right - from the description by the OP.