Maren...often it's not about confidence... some dogs just don't bite hard in "play", because of bite inhibition somewhere down the line, or genetics. If there's nothing wrong with his mouth, the following has helped me with a few dogs...but you won't get past their basic genetics: there are HARD biters, and there are NOT SO HARD BITERS.
1 - Back tie (6 - 10 ft) the dog with a shock absorber mechanism ( two bicycle tubes, bungees, springs, etc) on a harness.
2 - Make up a leather tug 3 inches in diameter by 2 ft long, and attach it to a sturdy 10 ft line.
3 - Make sure your dog cannot puncture the leather, nor catch his teeth on your setup.
4 - Frustrate the dog a bit, feed the tug and in a smooth and firm manner (NOT JERK), pull the tug through his teeth.
5 - Simply pull the tug through his teeth, walk away and leave the dog ALONE for 10 minutes. 3 times max per session. If the dog's pressure improves even a bit, release the tug and PRAISE.
6 - If in the 3 times there's no change in pressure, simply put the dog in his crate and go home. Try again the next day. Don't be talking to your dog while you're putting him away. Ignore him as much as you can.
7 - If the dog's pressure improves, start PROGRESSIVELY and PATIENTLY asking for more and more pressure before you release the tug and PRAISE.
8 - After a while, when you see your dog's pressure at a decent level, put the behavior "on cue", ie. say a word IMMEDIATELY BEFORE the dog bites the tug, so he can begin to associate the "word - bite - pressure - tug release - praise" sequence.
If you're patient, you may find your dog's pressure increase significantly.
All that said, you first need to find out - at a minimum - where your dog is bleeding from.
Hope this helps.