Going from all-you-can-eat alfalfa to only getting grass hay is a BIG and very unwelcome adjustment to a rabbit. I went through it with two of my girls when they reached the 6-7 month mark and had to transition to an adult diet. I tried - literally - 17 different hays (various brands and types) and they wouldn't eat more than the tiniest nibble of them! Then I found a place online (which, sadly, has now closed) that sold a hay mix that was 20% alfalfa and 80% orchard (where the plants were grown together and harvested together in the same field) and used that for quite a while before finally weaning them onto pure grass hay. Given your rabbit's age, I suspect he'll be very resistant to giving up the sweet, rich taste of alfalfa - you could try looking online for another retailer who sells orchard/alfalfa or timothy/alfalfa mixed hay. You could also try mixing a little alfalfa in with his grass hay, though I suspect he'd be able to smell the difference and would pick out just the alfalfa to eat if it hasn't been grown or at least stored together long enough for it to all smell pretty similar.
20% of his hay being alfalfa is far from ideal since he's an adult, and overweight at that... but I feel like it's still better than not eating hay at all. I think it took me about four months of feeding the 80/20 blend instead of pure alfalfa, then a couple more months of gradually phasing out the 80/20 blend for pure grass hay before my girls were 100% weaned off of alfalfa. You could also try 3rd cut timothy - 6 1/2 years later, it's still the only hay that Nala really devours (she was one of the two that I fed alfalfa to as a baby; Gaz, sadly, passed away about 5 years ago). 3rd cut is softer, sweeter and richer than earlier cuts so it's most likely to appeal to super-picky bunnies. Of all the grass hays, it's got the most in common with alfalfa. Earlier cuts are, theoretically, better - more fiber, less protein - but they're only better if the rabbit will actually EAT them.
