Boarding - never ever. It is not only a waste of money, but very hurtful for your dog, he could go out of it really agressive and I will explain you why and how the board training works.
Kennels, daycare centres, boards originally were created for people who go on holidays and in need to leave their dogs in professional hands. They always were as a sort of "dog hotels" where dogs of different age groups are cared and allowed to play with each other. Nobody would argue, that young puppies are missing their sisters' and brothers' company and play, I really appreciate professional boards for puppies, these people provide basic training with puppies under 6 months as well. But that is all positive about it. From one year old the dog gets into sexual maturity and starts seeking a status in his pack, what is his human foster family. Young dog is in desperate need of a leader, especially a young male, and the sense of a pack unity becomes very prominent. Separation from the owner normally causes a terrible stress. The board uses this stress in their training, frightened and stressed dog is easier managed. They would use prong and hunger to "correct" your dog and feed and fondle him just before you get him back. But nobody would allow you to see HOW he is trained. The trouble, that it doesn't work, only your dog gets over the stress - he would be back to squire one, behaving as before. What will you do? You will call back the board. They will tell you, that your dog need more corrections (more money out of your pocket). If your dog has enough of spirit he would get truly agressive, and if you tried to repeat the trainer's methods - he might bite even you. If he's a weaker nerve - he would be scared of you and the prong, you can lose your dog's trust forever.
A good trainer never traines just a dog, forget Cesar Millan. A really good trainer never touches the dog of another person, as he traines not the dogs, but people who have them. You have problem with keeping your dog's attention on yoursef, that is purely practical matter and no written advice can help you. Make your research for a good personal trainer, preferably with a dog working as his assistant, dogs learn good manners from other dogs much faster.
We had a saying written on the board in our club "An obedient GSD is a tired GSD". Your dog reactivity could be easily explained, lack of physical exercise is a very common thing which leads to frustration and high reactivity in young dogs. That fact that he bit two dogs without much reason only proves it, he feels himself a Yuri Gagarin, but, alas, there's no space to run through, so, he gets angry and blames strangers and other dogs. If he walked back home barely dragging his legs after chasing the ball half a mile down the hill and back up for two hours - I doubt he would bark at all. You need to let this vapour out.