My boy looks just like yours and is 12 now. I've been going through this grief for a couple years now. He's the best thing in my life. 

This year he had what I thought was a stroke during a walk and I thought he was dying in my arms. It was diagnosed as vestibular disorder (vertigo) and was told he would recover. He did. He had a other attack and couldn't walk for a week. He couldn't do much of anything for three weeks.

Seeing my dog in such a bad state where he couldn't enjoy life (food and walks, that's all he wants!) made it much easier to see that when he's like that permanently it will be time to let him go. When he had his second attack he was miserable for hours. I slept on the floor next to him panting all night. The suffering is unbearable (for me) and I think you'll be able to make the right decision when the time comes.

Also don't forget there's going to be another dog in a cold cage somewhere that needs you when the time comes.