Hello there. I would be aiming for one change at a time First thing would be to either wean across to timothy hay or pellets (the veggies can wait). If you’re planning to try going pellet free, I’d suggest switching the hay first. Chances are, that he’s not going to want to eat the timothy so you’ll have to continue to provide alfalfa as well, but mixing it in may at least make him get a taste for the timothy and over time you can decrease the amount of alfalfa that you mix in providing he’s eating the timothy hay well. I believe there are three different cuts of timothy, 1st being the coarsest and 3rd being the softest (from memory) I think most people feed second cut but he’s partial to soft or hard stems you could try the first or third cutting. Timothy hay is not your only option though, you could also try orchard grass hay which quite a few people really like too. So changing the hay would be my first choice. If he refuses to eat the Timothy completely, you should still provide some alfalfa, because he does need to be eating hay, and alfalfa hay is better than no hay at all I think. Some people even spritz a new type of hay with apple juice to help entice their bunny to eat it, but you need to be mindful to change it completely daily so that it doesn’t spoil. Even if you’re not doing that, changing hay daily helps to encourage eating because it seems more “fresh”.
If you want to go pellet free, I would then reduce the pellet quanitity (how much is he on?) to max 1/4 cup per day while you slowly introduce greens. As with any dietary changes for a rabbit, it’s one thing at a time in very small amounts slowly increased over time. So once he’s eating a grass hay without problems, introduce your first green, and give only a very small portion, something the size of a coin. If you don’t see any stomach upsets, you can double the amount the next day, and so on. After a week with no upsets, add in the next green in the same manner. By introducing one at a time, it will enable you to tell which greens he responds well to, and which he doesn’t and exclude those from the diet, or introduce them later on once his gut flora is better established. Some greens that usually go down well first up are coriander/cilantro, basil, mint, and small amounts of dark leaf lettuce.
Once you have him eating a good variety of leafy greens and veggies every day that has a nice balance to it, then I would slowly remove pellets from the diet. Since they essentially contain all the nutrients that wouldn’t be obtained without a balanced veggie diet, you don’t want to remove the pellets before you’ve got him onto a good rotation of greens. I know they’re alfalfa based, but given how long he’s been on them, a few months more of a reduced amount while eating a grass hay shouldn’t do any harm.
So to sum things up, do the changes one at a time. I think the hay is probably the most important to begin with, and work from there. Hope that helps
Edit: Additional. If after he’s eating greens and you’ve taken him off pellets and decide you want him on pellets again. I don’t think it would be too hard to then introduce him to a timothy based pellet after he hasn’t had pellets for some time, because they’d be like a treat then. Just a small amount at a time though of course