Being offensive is his act, he saw a niche in the market and went for it - this is how he got his huge success. Lots of punters like this sort of stuff, Chubby Brown is never out of work, nor was Manning. All he's had to do is change the racist gags into ones about other minoritiss and individuals he doesn't like, to keep his growing laddish fanbase happy.
But it's lured him straight into the hot waters Bernard Manning was caught in, and Manning was far more confident in defending himself, it appears. Boyle seems to get flushed over with immense anger that anyone should criticise his right to free speech, whereas Manning very calmly but firmly stated reasons like the treatment of our troops in the war, when he made jokes about Japanese, or about our culture having to make way for other cultures because 'they come over here to live and bring their country with them.' Many disagreed but you have to say it was some sort of logical defence and Manning really believed it. In that respect Manning was a very honest comedian, and there were many things he would not joke about, and I don't think he would done a routine about DS.
By making fun of disabilities the only defence you can have is, 'But it's true though!' as the angry Boyle is quoted to have said. That is no defence at all, it is a cop out and it makes Boyle look heartless, without any care for anything - he's a father himself! Boyle has pushed the boundaries of what you can do comedy about to beyond stretching point now. He's brought the topic of a law to stop offensive material back into the spotlight. He's just giving fuel to the PC loonies that want to curb what comedians can joke about. And C4 I expect are starting to regret signing him up, disability jokes are not what Channel 4 is about! It'll be interesting to see if continues with that routine on his tour, because a lot of eyes will be on him now.