Something Wrong With my Dog

[overdose]

Asalamu 3laikum

I bought boxer dog two months ago,he's a male,active, muscled built when i got him.
a month ago he stopped eating royal canine or pedigree food,,I tried to mix them but it didn't work,anyway I changed his diet to chicken breast and he loves it,,but i noticed that even though his appetite is masha allah very good he loses weight soo fast and even stopped doing the normal things he used to like playing and jumping etc,,I went to the vet clinic the doc gave me some medications but still he looks sick

is there any diagnosis for this case??or medications it's serious matter

thank you >>