Hey, Hugs, welcome to the boards. First of all, I don't think the doctors even know what to do with post-menopausal females, let alone young males

. That said, what I can tell you is that you may well end up getting a lot of great info right here! You had asked if exercising might be dangerous for you. Some will disagree, but I say just go ahead and do whatever exercises you used to do! I have a lumbar BMD of -3.4 and I downhill ski, lift weights and hike.
I'm curious: is there anything that could have caused you to have decreased density (thyroid/parathyroid disorders, steroid use)? How did you end up having the DEXA scan to test for density.....did you fracture something? If you have a systemic condition that influences bone density, it's best to treat that ASAP. Otherwise, if you're a healthy guy who eats right, works out, and gets the proper vitamins etc. you probably shouldn't buy into the marketing of fear. There is an excellent book you should read called "The Myth of Osteoporosis" by Gillian Sanson. Her son and husband both have extremely low BMD and she will give you a totally different perspective from the mass hysteria that we've come to expect from the media and the health-care profession.
I've had plenty of specialists try to scare the living daylights out of me, but thankfully my family doctor's opinion of osteoporosis goes something like this: "sure, your numbers look pretty bad on paper, but there are many other factors besides bone density which influence fracture risk".
Hugs, take your time with this before you decide to do anything-or nothing at all. Do some research, read some posts, and don't let this consume you. You're not going to fall apart overnight. Thanks for posting; it's nice to hear from some younger people-males too-in the same boat.