My 17 year old cat has always been thin but has very suddenly lost a lot of weight this week, we noticed on Thursday night and as recently as last weekend he seemed fine other than being a bit stiff in his hips and sleeping a lot but he has been like that for a long time which we assumed was his age (we also have an 11 week old baby, a toddler and 2 other cats so we likely missed early symptoms

Does anyone know what might be wrong? If it is something that can be treated (without the treatment itself being more distressing than his current situation) I would want to but I fear that taking him to the vets will just be traumatic and they will say that there is nothing they can do but put him down, if the end is near anyway I don't want to put him through the upset of the vets trip and for this to be the last thing he knows

Many thanks in advance,
Becca