Avoid the spine, but other than that, anything goes. Of course, don't dig into his kidneys. You can touch bones but there are some places and moves that make people feel uncomfortable when you massage, like moving the shoulder blade in a funny way. It's still good to massage on top of it or beside it though because there are muscles that get really sore in there. Practice helps.
Cut down or file down your fingernails before you massage. Any kind of massage oils are nice, but good ones are grapeseed oil or sweet almond oil. You can get them at any health foods store for cheap. They have no scent (you can scent with aromatherapy if you want), and are completely natural ... natural enough to cook with.
Start with long, flowing strokes up the back (again, beside the spine! never on the spine!!) with the palm of your hand. Keep your fingers on the skin, don't roll them upwards (like, your fingers rolled up towards the ceiling when your palms are down), it feels nicer. Do that a few times, then use your knuckles. Then, your forearm. Trust me, it feels wonderful, but when you get towards the upper back, aim your knuckles and forearm between the shoulderblade and the spine. It takes some practice. Don't worry too much.
Always ask how they like the pressure. Not like a parrot though.
Kneading is nice. Knead using your knuckles for big muscle areas. Start at the lower back and work your way up. Try to move nice and slowly. It's good to have soft, flowing music that helps you move sloooow... Also, you can go in deeper when you move slowly. But, NEVER go in deep unless you have worked a muscle for like 10-15 minutes. You might not wanna do that anyway lol.
Knead your way up the back. Switch from knuckles to palms or fingertips midway up. Like when you start hitting ribs. There's a nice area to massage that is between the spine and the shoulder blade, called the rhomboids (thats a muscle group), that tend to be sore in most everyone. It's nice to massage them with kneading thumbs. Move from the spine to the edge of the shoulder blade, not the other way. Rub your palm over it in circles.
Don't be afraid to do the same thing over and over again. Now, if he likes 'deeper' massages (some people like it so it HURTS... some are wimps and like it nice and gentle hehe)... A muscle will be warmed up if you have rubbed it (circling, kneading, etc) for about 10 minutes. You will know when the area is red. This is a good sign. To go in deeper: Place one thumb over the other and slowly move the thumb. They will tell you (or you can ask) if the pressure is too hard, too soft. Some places are nicer for this than others but I don't think you want a book.
I hope that helps. Sorry for rambling!