I rescued a great dane a couple years ago. he was from an irresponsible breeder situation and had been abused and severely neglected. He has all kinds of issues. The first thing we did was get him neutered and worm free. he was loaded with worms and skeletal. Today he is shiny, worm free, is up to date on all his vaccines and routine vet care. However, no matter what he is still skeletal. My vet has tested his thyroid and it's normal. He seems healthy and happy in every other respect but looks like I don't feed him enough. I've tried everything from EVO to the Barf diet. He's currently on a fatty acid coat enhancer and an enzyme medication designed help him process food. He used to simply not eat enough but I finally have him eating more and he just doesn't gain no matter what I do.

This is acually rather common in Danes.
Do a google search for "Satin Ball recipe" I add peanut butter to mine for some extra calories and taste. pets question and answers,www.5d2d.com
I showed Danes for a number of years and routinely used Satin balls when on a show circuit. (One or two tennis ball size)
Once weight is where you want it to be ... just give smaller balls about every other day .. feeding regular food on the off days of course. They WORK!

Best of luck to you ... I guarantee Satin Balls will put weight on fast!