Best way to put weight on a skinny fish?

[ACBlinky]

I bought this guy knowing he'd be a bit of a challenge, but couldn't resist that pretty face. He'd been in captivity for several months and was eating, but was either not taking much food or was being fed substandard fare because when I got him he showed signs of HLLE starting, and I could just about count his ribs.

At first he was picky, only taking a few bites of something before he'd lose interest, but now he's eating well. His favourite food ATM is Hikari Marine 'S' pellets, and ironically his least favourite is frozen Hikari Mega Marine Angel formula! There's nori in the tank for at least a few hours a day (I put it in the clip in the morning and it lasts until the fish/turbos finish it) as well and he's brave enough now that he's started picking at it.

I'm just wondering if anyone has come across a great food or feeding method to put weight on a fish, or if the best thing to do is just continue with multiple daily feedings of frozen and pelleted foods.

Here are a couple pics. The little pits in his head appear to be receding, and believe it or not he's fatter than when I got him in early January:

 

[marrone]

Try feeding his some mussles or clams on the shell. This is a tough fish to keep and most usually die from either not eating or not eating the right thing to keep them alive. The smaller ones do better but even then most don't make it.

 

[ACBlinky]

Thanks for the suggestion, I'll try that!

He was purchased by someone else five or six months ago, and I don't think he was getting the nutrition he needs for the four months they had him, hence the weight loss and little pits in his head. They ended up returning him to the LFS where I work, which is when I bought him. He's just a little guy, about 2" long, so I'm hoping he'll do well given proper care.
Right now he's eating brine, Hikari Marine 'S' and Marine 'A' pellets, mysis, 'marine formula', Hikark Mega Angel, plankton and nori (listed in order of his preference). He also picks off the rocks all day long.

I mainly posted because although he's putting on weight it's slow and I wondered if there was something I hadn't heard of that would help. I'll see if I can get a clam or mussel at the market tomorrow - I'm assuming it needs to be opened for him, which means it'll be dead. I hope the fish eat it quickly before it makes a huge mess!