It is often easier to transition a bun from alfalfa pellets to timothy pellets, so you could start with that transition if you haven’t already. They only need one source of alfalfa when they are growing. They also sell pure timothy pellets for horses, you could try to incorporate those as a way to boost his timothy consumption in the meantime. Not sure of his exact age, but if he’s over 6 months you could start reducing the amount of pellets fed.
Sometimes it just takes consistency and patience. If you keep mixing the alfalfa with grass hays (Oat is another good one to try), eventually he will likely take a nibble. Then you can start slowly reducing the amount of alfalfa and increasing the amount of grass hay.
Fresh grass is a great idea as well, you can grow cat grass/wheat grass for him if you can’t find a good source outside.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.