Injectable Vitamin B Complex (including Vitamin B12) and injectable 'Liver and Iron' (sold for baby pigs) are both great to increase appetite. We use them both all of the time when a horse is not doing well and won't eat good.
We give a mature horse 5cc of each and just squirt it in their mouth. If I did give it in a shot, I would give it IV, but it works almost as fast giving it orally.
If you give 5cc a week of each, you should see a big difference withing a couple of weeks. If you don't, then you have something bigger going on that needs to be found. A full blood panel and a really comprehensive exam would be called for.