Wow he looks nothing like a stallion! He has no muscle, or crest and like you say, no spark... he just looks... sickly. I think a vet check is important here, gut instincts are good haha. He may be lathargic because he has no condition. Try to look into his past, ask owner for photos etc to see if he has always been this way, or if its a more recent change.
It could be do to with worms- encysted (sp?) worms don't show up on egg counts but have the same effect on the horse, and like someone else said, wormers don't always work. if his teeth are bad get him sorted and see if any improvements start showing.
Like others have said, vet check asap so start convincing the owner lol
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz