Anytime that you switch pellets, you are supposed to mix some of the new in with the old. I believe that you start with 75% of the old and 25% of the new, then gradually change the percentages until you are feeding only the new. Can you pick the colored bits out of the alfala pellets so that you can still use those pellets? One cup of pellets is too much for an adult rabbit. By 11 months, he should be given amounts according to his weight. There are different guidelines, but one says to feed 1/4 cup pellets for a 5 – 6 pound rabbit, then increase or decrease the amount if the weigh more or less.
Orchard grass is one of the sweeter hays, but you might need to try some other varieties to see if they tempt your rabbit. You could try a light sprinkling of the alfalfa on top of other hays or use some hay toppers like they sell in the BB store.
Yogurt treats should never be given, but you could give him small pieces of fruit, such as apple or pear, just don’t give him much. If you limit his pellets and keep a good supply of hay, possibly a variety of hay, he will hopefully start eating the hay and timothy pellets.