Well......I've found a couple of things will make a difference.......
Vision of course very well be a problem....However I offer something I've found through teaching.....
Ever shoot for score?....Not counting natural point of aim and brass and such....your trigger finger never goes to the first knuckle.....instead you put the trigger on the meat of the end of your finger......or when you squeeze the trigger you'll pull your shot to the right...a 1/16th of an inch over 2 or 300 meters is several inches
With that said where is his thumb? Is the stick back on his thumb near the knuckle? You know back on the pad of his thumb so to speak.....if so when he relaxes his hands, his thumb will pull the stick back...causing the airplane to climb.....
Get the stick nearer the end of his thumb...and get his thumb knuckle higher...so when he relaxes it's not so natural to put back pressure on the stick....
Where is the airplane trimmed at? I mean what throttle setting, if like me you trim at half throttle....If he flies at more than 1/2 throttle it could cause a climb...so with the thumb and the trim/throttle it could cause the climb.....
Good Luck