D.K.
Your husband needs to realize that at this age, they really need to be allowed to start making some decisions (and mistakes) for themselves. Personally, I agree with your son about the long pants thing. A jacket/sweatshirt should be adequate on a cool night to serve until you get home.
Yes, you should back your husband up....on the important issues (like how old he has to be to date, can he get a mohawk, can he pierce his nose, can he use his college fund to buy a car instead etc..) but not about little things like this. Your son needs to grow up knowing that spouses can disagree with each other and handle it constructively without it always having to be a fight. Model for your son how to civilly disagree and debate and do it with the small stuff. If anything this will reinforce for him that there is NO negotiation when you present a unified front on the larger issues.