Training wheels generally don't "work".
Kids generally don't have the capacity for mental separation to use the trainers "only" to keep from tipping over.
They ride training wheel bikes as if they are chain-drive trikes.
Balance/straddle bikes work far better.
Once they manage coasting on them, transition to pedal power tend to take 15-45 minutes, with a minimum of tears.
WRT "hard on flat ground" - inspect the bike. The cheap way of building kids' bikes is to use as many adult-sized parts as possible. I've seen a few with 170 mm cranks. Riding a beast like that is indeed a rather awkward experience.
Gearing is less likely to be an issue due to the whel size, but can be worth checking out.
At the end though, if the kid isn't interested, it's difficult to force an interest.
Is there something that he likes that can be used as a lure?
As in "let's ride to the xx and get an ice cream/go fishing/watch the "whatever"?