I think we need to start our own thread for long-haired babes, here on BB - a long-haired baby buddies group!
If you don't want to get it cut, don't. Is it really going to make other people feel better if it's cut? If so, why is it that their happiness is so precariously hinged on what your DS's hair looks like? Wouldn't it make it THEIR problem?
People soon got off our case after starting on the 'oohh, it's time he had a haircut' and I tartly replied 'no, he doesn't, and we're letting it grow long'. DP doesn't want it to grow long but is willing to go along with it because he knows that it's not worth going against me on it...I'll be devastated, and I told him so! So then he reverted to the 'but it will get in his eyes, can we cut his fringe?'...ummm, no! Now that MIL hasn't taken to his fringe (again) with scissors, it's a nice length to tuck behind his ears...and, admittedly, she didn't cut so much off that it was noticeably shorter at the time (not the point, though - she hadn't asked me).
Yes, if you cut the hair, it may grow back changed...not necessarily in a way you'll like. One reason I'm not touching DS's hair is that there is no guarantee it will grow back curly. Some 'experts' say that it's just the weight pulling the hair straight from the top...what if it's not?
As for looking like a girl, I just say 'he doesn't care what he looks like ATM, he doesn't have such prejudices', especially as wherever he goes there are always more people who coo over his hair and say how much it suits him than people who shake their heads.
The people who know him and love him won't care how long his hair is. The ones who don't just don't matter, do they?
Final note - 100 years ago it wouldn't have mattered about long hair on boys, for some reason, adults feel threatened by little boys who aren't blatant ambassadors for all they hold dear about their version of masculinity!