PlanetCatfish.com

Hi I am from Stoke on Trent and I have been keeping and breeding Plecos for about five/six years.

I purchased a Magnum L047 on Sunday and never realised until I got him back home that he is very lethargic and has a sunken belly. This morning I have noticed that his eyes are beginning to sink in too

My husband has always wanted one of these and it was a present for him, is there anyone who can please help me get him back to health.

Please help

The prognosis is not good.
A fairly delicate species as this which is so far gone due to starvation and/or parasite load is usually going to die. There isn't any specific treatment which is likely to be successful.
Just providing good care and careful feeding in a quarantine tank is probably your best hope. I would probably treat it with flubendazole and praziquantel while the fish is in a quarantine tank. Those will help the fish fight off the parasites and having no competition until recovery is best as well.
Good luck.

Can you return him?

Unfortunately he died
I tried to speak to the person who sold him to me and he refused to accept responsibility even though his stomach was sunken when I had got him home. I wish I that I had put him back in the bag and took him back that day but I didn't and the seller said it was not his fault even though the fish that I brought at the same time died before I even got to the car, I should of given him both back then!

Sorry to hear of your loss, Baryancistrus can be difficult to acclimatize and as a previous poster said probably too late from the signs you described, if you decide to get another one at least you know what to look for, I always keep some stones that I rotate from tank to outside and back to tank in some water to get a natural biofilm/algae for any new arrivals.
Was this a shop or private purchase as I am often in Stoke-on-Trent and would like to steer clear of this seller, no need put it in this post but maybe you could PM me, Cheers.