You can feed both alfalfa and normal hay. Alfalfa isn’t sugary, it’s high in protein and calcium, and that’s why it’s not recommended for buns over 6 months of age. But it’s excellent for a baby bunny. Baby bunnies need the protein and calcium to grow.
But it’s also good to give a little bit of grass hay as well, because you want your bunny to get healthy eating habits from the start, and grass hay is the healthiest food for a grown up bun.
Normal pellets won’t hurt the bunny, but if you can get alfalfa-based pellets, I’d choose that for a bun this young. If you can only get “normal” pellets for adult buns you can just feed a larger amount.
At 5 weeks a bun should ideally still be with his mum and siblings, so try to spend as much time with him as you can. Monitor his poop rather closely – his tummy is extra sensitive because he’s so young. At 12 weeks you can start introducing greens.