i believe you can get some form of seeweed from the grocery store (i still need to look into this myself)
other than that you should be able to get food from LFS or online
frozen cubes, pellets, dried sheets of seaweed. if not eating try soaking his food in garlic (not garlic salt but real crushed and mashed into tiny bits garlic) maybe pureed is that the right word lol. i soak my frozen cubes for 10 mins then get any large piece of garlic out before feeding the cube.
not only will garlic help his immune system it may also entice him to eat.
online im sure this site sells food, and theres thousands of other sites on the web to order from as well.
first got to get that guy to eat. go to LFS and get some frozen cubes of algae/seaweed he will probleby jump right on them.
also get a veggie clip at LFS and the dried seaweed put a 2 X 2 square on the clip and suction it to the glass he will take awhile but when he realizes what it is he will pick at it all day long.
Good luck hope this helps.