Or you can tell him that he will have to do better in training if he wants to remain with the club. Usually that gets players to back off--unless their training load really is to high. If they are still on a conditioning training schedule after the season has started, for example.
In my practice 80% of player have such complaints. Usually I ignore them unless it concerns players those are really vial and indispensable - in order to avoid injuries. Mostly I ignore such complaints and have the advantage in training progress of my players.
It's a damn nuisance this is and the only way I can see around it is to have a different training schedule for every single player.
e.g. - I have three centre mids all on training for centre midfielders, one moans the workload is too high, one moans it's too low and one's perfectly happy with it so unless you sat there for a week and made an individual scheme for each player in the squad I don't see how you can stop it. In my opinion the training is yet another feature that needs serious rework, it's boring and there's very little to suggest what, if anything, is working apart from the red or green arrow thing, I don't think the training works particularly well at all.
as in some big training thread here on the forum, someone suggested the best way to make your training is to set every slider on half and heavy traninig, and adjust the sliders every 2 month given the results in the training tab, the bigger the colum, the heavier the training for the scala should be because his potential in this area is the most, and re-do this every 2 month for every single player takes like at least 15 minutes if note more, and yes its insanely boring as well...
This feature is a bind! If they insist on having it they should give us a button that says "Ask trainers to fix this problem" instead of the manager having to go into the players training page & fiddle with the settings.
This is what the idle trainers should be doing anyway & if they did their jobs right, the manager wouldn't be getting these complaints in the first place. Grrrr!