Dog Heartworm Treatment

I have recently adopted a 6 month old pug from a lady who was struggling to care for him (as she was at work all day and he was left alone). As soon as I got him home i noticed he was unsteady on his feet, seemed like his rear end was especially weak. He could still jump up and had a lot of energy, but seemed to sway when standing still and would sometimes fall when turning quickly or landing from a jump.
I waited a few days and took him to the vets for a health check (which was pointless as he was hardly looked at) and whilst there i mentioned his legs. The vet told me to keep an eye on him over the next few weeks, and if he was still unsteady to bring him back.
I’ve been watching him carefully, and he still seems to be quite weak on his legs…..when climbing steps or simply jumping up the curb he will fall over. I was thinking it may be because of a lack of exercise with his previous owner, maybe his muscles hadn’t developed yet. I’ve been taking him out a lot, and feeding him on healthy food, but he still seems unsteady. Alongside this, he seems to have a lack of co-ordination. He will respond to calls and sounds sometimes, but will often turn in the opposite direction to where the sound is coming from. He also struggles to find his food, he will sniff around for ages before finally finding it (even when it’s right under his nose he will sniff off to the side). His eyes seem to be working fine, he knows where he’s going anyway and notices things in his surroundings.
I’m just really worried about him, he was sick today for no apparent reason (he wasn’t fed any treats, just his usual food…apart from a worming tablet I bought at the health centre). And is proving difficult to potty train….after just being out he will often come in and pee on the carpet right away.
I’m praying he hasn’t got a crippling disease. He’s an adorable dog, and still only a puppy. He does look really small for his age, and his legs look a lot thinner than they should. I’ve only had him for 5 days.
Has anyone experienced these same symptoms with their pug??? What was it, and was he ok??? I’m taking him to another vets next week if i don’t see an improvement!!! Please god let him be ok the poor thing!!!