I was able to make one of my budgies eat the pellets by using a mirror. If you put a small mirror on the food dish, your budgie might think that the "other budgie" is eating the pellets and he might decide to taste them.
I think it could use this trick to make him eat fruits and vegetables too (but you should limit fruits because of the high sugar content), since you said that eating them in front of him didn't work.
And perhaps it might work with the seeds he doesn't eat too, if you are able to remove the one type he eats. It is certainly not healthy for your budgie to only eat that one.
The presence of the mirror in the cage should obviously be temporary because mirrors are usually bad for birds (they can get aggressive or obsessed with their reflection).
I hope I helped you.