Instead of trying to make your son not shy, maybe you need to first accept that he is shy. It needn't define him, but it is his first and most natural way of interacting with the world right now. Don't try to change him with playdates or compare him to his sister. Let him know it's okay to be shy. Let him know it's perfectly fine to enjoy playing alone. If you are out someplace and he expresses an interest in talking to another child or playing with a group let it be okay to ask you for help. You can certainly approach a group or child at the park (or wherever) and say "This is Josh. He's feeling kinda shy because he doesn't know you guys, but it looks like fun what you're playing...can he join in?" He may find it easier to meet like-minded friends at an extracurricular team or club or class that he has an interest in. When the kids are all working together as a team or on a craft, etc. they can get to know each other in a safe and slow way, week after week.
I have three little boys, and my oldest is 7 and we have had very few play dates with school friends and no sleepovers yet. Although not shy at all, my son plays really happily alone with legos, etc. all weekend long. I feel lucky he's not begging for friends over yet! I feel like I have a houseful with just my own kids right now! :)
I think if your son really wants to be alone you should be supportive of that. If you are able to let him know that it is okay to be shy (even a good thing, for example, he will likely always take the time to think before he speaks...what a great gift that is!) he will do things and be social in his own time.
Last thought (I'm rambling, sorry!) maybe a good match for him right now would be a friend who's a girl. She would maybe be more sensitive and be a better listener in a friendship than another 8 year old boy.
Embrace him as he is. Look for all of the wonderful qualities his shyness brings your family and those qualities, although certainly not the whole of what he will always be, will serve him well as he navigates the world.
P. (mom to three boys ages 7,5, and 3)