My big boy (German Shepherd) isn't looking too great

He's not been right ever since he had the fatty lump removed from behind his front leg (and it taking 12 weeks to heal didn't help

) he's losing body condition, seems really breathless and just last night his legs/back end seemed to give out on him a couple of times when he was trying to get up. He also seems to be going deaf and just looks so sad all the time

He's just been wormed and spot on'd (it wasn't spot on, but it was a topical flea treatment) does anyone know if this could be a reaction to the meds ?
He's only 10 and a half and right up until 3 or 4 months ago was in pretty good shape for his age, but now he just seems, well...........done

He's not in distress and eating as normal, but he's just not his usual lively self.
I'm going to phone the vet on Monday, but I have always said that when his back end goes (as is so prone to happening with GSD's) I would have him put to sleep to save putting him through the trauma of endless medication, operations and stress - but now that I'm potentially facing making that decision I just don't know what I'll do without my hairy baby

Here's a wee photo for anyone who's not 'met' my boy - positive vibes requested for Boris please

Karen x