In order to tell is a sheep is fat enough for slaughter you need to feel the top of the tail, just before it joins the body.
Once the main body of the animal has developed a certain ammount of fat covering, the fat starts to develop on the tail bones, starting at the top.
So an animal is ready for slaughter when you can feel the fat filling in between the individual spinal bones of the tail, so the bones actually feel padded, as apposed to boney.
You would be better getting an experienced farmer, or even possibly your sheep shearer to show you.
As for your ram - I would take him in now anyway before sheep prices drop. If you've had him on good spring grass he should be ready and if he's not then he never will be, as unlimited spring grass is unbeatable for finishing stock. So you've nothing to lose by taking him in now as a Friesland x Torddu is never going to be the most desirable meat animal in the market. While you're at market get some helpful person (there'll always be someone happy to demonstrate their knowledge!) to show you how to grade a sheep.