You might find there is a point that forces your hand. One of my boys had a stone removed last year. It had obstructed his urethera so he couldn't pee at all and was screaming in pain, it was the most awful thing I have ever experienced, we took him to the out of hours vet ASAP and he was operated on. The recovery was difficult for the first few days and he needed a lot of heavy pain relief and syringe feeding but his situation was more complicated as it turned out he had two mammary tumours which were removed at the same time so not just a straight forward stone removal....
It sounds like the stones are causing him pain if he's not eating so treating the pain is important and one of those options other than medication is to operate but if the stones move then you might end up in the situation I did where it's an emergency and it has to be done. That isn't a fun place to be and if they get lodges in his urethra or penis then they can be almost impossible to remove and you have no options at all then.
He is young and the longer you leave it to operate, the less fit he is going to be by the sounds of it but at the end of the day, you know him best and only you can weigh up the risks of the operation and anaesthetic compared to his quality of life. But to weigh all this up you need a good, honest chat with your vet.
Feeding wise try using critical care mixed with warm water or pureeed veg baby food to get plenty of calories into him. Feed as often as you can and a good amount each time. There's an excellent hand feeding guide on the forum somewhere....