He doesn't look too bad, weight wise.You have to be careful with underweight horses, you still have to choose your feed carefuuly, even though you want them to gain. The sweet feed may have had a lot to do with this, so I am glad you have stopped that now. I am glad that what you are doing so far has shown improvement, but please do bear in mind that farriers are not really qualified to offer medical advice. I wouldn't panic, but you do want to get a vet out to take a look, as the above poster mentioned there may well be other issues. Did you have a pre-purchase examination with a vet before you got him? If you didn't it might be a good idea to get him properly examined since he is new with you. The vet will also know more about feeding than your farrier.
Again, I am sure you will be responsible and I wish you luck with helping him get better.