How cute. I too have a boxer and they are such adorable energetic creatures. Will definitely help you through this. It is not that hard, but requires some patience, that's all :)
Firstly, remember the basic principle: Your little puppy just does not have a concept of right and wrong. So the basic idea is to make him so happy every time he does his potty in a particular spot that he just enjoys doing his potty in that same spot.
Puppies usually want to do potty just after waking up or after meals. So next few times about 15 minutes after his meal take him to his designated spot. The first 2 or 3 times he might not understand why you have taken him there. So he will resist. Just don't give up! Stay in that spot till he does his potty. The first 2 times, I had to wait for 1 whole hour with my little puppy. Finally she could not control any more and did her potty :)
As soon as your doggie does his potty in the right spot have a great celebration. Call him good boy several times. Run up and down a little with him. Clap your hands. Do what ever makes your puppy very happy. If he likes treats give him a few. Trust me, after 3 times of this, he will begin to feel so happy about doing his potty in the right spot.
If he does potty in the wrong spot, its ok. Just say a stern "No, Bad boy". Say it only once. Clean it up and cover with disinfectant, to remove the smell. MOST IMPORTANTLY: Don't over react. Don't get angry and yell or scream or punish. For a dog, this is attention too. And your attention is so valuable to him that even if you are yelling, he will crave your attention and continue to do potty indoors.
Remember, you want your dog to think "Oh! If I do potty in here nothing happens. But if I do potty out there, these people have a party and its so much fun. I want to potty out there all the time".
In this method of training you too will be happy. You don't get frustrated. When we have a potty-party, we too end up getting excited and start laughing and just find our puppy so cute when she gets so excited :) It's fun! You just need patience the first 2 or 3 times, that's all.