This is a tricky one. With a disabilty that can affect the quality or timeliness of your work, I personally I found that it is often advantageous to tell your boss so that he knows why something is happening. However, in a profession that requires frequent learning, it might be considered by some to be a something that disqualifies you from the job (which is why you usually don't bring it up in an interview). Further the personality of the boss and corporate culture come into play. If you can see that they have made obviously accommodations for someone else who has a disability, they are less likely to react badly. Only you can truly assess how risky telling your particular boss and your particular company is.
You should also assess why you want to tell him. If you are telling him because you want to be assigned to work on legacy stuff so that there is less learning involved is different than telling him because you think you are about to get a bad performance appraisal.If you don't have any accomodations you want and have no performance concerns, then I would not bring it up.
If you choose to tell him, then the way I would approach it is:
First make sure your performance is a good as it can be. People will make more accommodations for a valuable employee than one who is average or below. So go the extra mile for some time before you bring it up and make sure he knows you are going the extra mile.
Do not bring it up as an excuse if he brings you in to talk about a performance problem. That is the worst possible time to talk about it. If that happens, accept whatever criticism he gives you and resolve to try to fix it. Then about amonth later, come to him and tell him what you have been diagnosed with and follow the rest of the steps. If he fires you for not being able to keep up before you have told him, then anything you say is too late at that point. In that case, what you want to do is look for a position where they work on less cutting edge stuff, so that there is less to learn on a daily basis.
Have a plan of action. "I have a learning disability, you figure out what to do about it" just isn't going to make it. Be prepared to discuss how you personally work around your disability (and highlight your successes) and what accommodation in the workplace you want. Be prepared to show how you spend your own time learning to make up for the slower pace of your learning at work, for instance. Also since this is a medical condition, you might want to provide documentary proof that you have it. And you might want to be prepared with some educational materials; this is something your boss may never have heard of and not be aware of the the ramifications.
No matter how you prepare, it is risky, so be prepared mentally to have negative consequences before you do it. Even if you don't lose your job, sometimes, having it known that you have a medical problem that seems a bit odd can make people act as if they think you are lying. I had a close friend who got diagnosed with Chronic Fatigue and her doctor told her she had to reduce to part time. For the next ten years until she retired, her co-workers hassled her because they didn't believe fatigue was a real condition. Your condition could be similar because this is not a common condition.
