Poor fellow! Has you AV recommended anything? The collar sounds as if it interferes not only with his reaching the food in his dish, but also with holding pieces with his foot as he eats. If so, he might really appreciate assistance, in the form of you making sure the pieces are small enough for him to eat in one bite, and perhaps handing them to him one at a time. Yes, it's time-consuming, but it certainly will help your relationship!
My first thought would be almond or winter squash, because of the calcium content, and the fact that they are attractive to most birds. But od get vet input before starting anything different from your regualar diet.