Does he eat any fresh foods, veggies? If so add some pellet dust to that, just a sprinkle. it takes time but persevere.
PS Just remembered that I have put a food pot attached to the side of his cage and this has helped with making pellets more accessible to him.
He definitely prefers this as he will regularly go and eat some now, when he wouldn't before. BTW Plum not very keen on Harrison's although I have some and keep trying, his favourite is TOPs.