My son Is now 26. He is very disrespectful to his father and me. When he is talking rudely to me and I get on to him he just says shut up mom. Is it wrong for me to think that when his father is standing right there and hears this that he should say something? He always just says what good will it do? I have been trying since he was 16 but it is just getting worst. My husband says to just leave him alone. Is that really what I should do?

If that is not working for you - then no, you shouldn't just let this behavior continue. Does your son live with you?
I know that when I was growing up - if we disrespected our mother - whether my dad was in the room or not - we got in trouble.
You shouldn't let anyone disrespect you - son or not.
And your husband should tell your son that he is not going to talk to you - his wife - that way.
Would he allow anyone else to do that? If the answer is no - then your son shouldn't get a "free" pass!

So don't invite him over, ask him to leave, if he lives there, kick him out.

Your son is 26 and is an adult. You should talk to him as you would any adult that is treating you rudely and tell him that if he doesn't start treating you better your going to have to not see him as much and if he still lives in your house then you need to recommend that if he doesn't treat you better perhaps it would be best if he got his own place. I know it's hard to tell your son this but if you don't he is just going to continue treating you badly.

I suggest you become assertive with your son. Being assertive does not mean being aggressive. Assertive behavior is not hostile, blaming, threatening, demanding, or sarcastic. Assertiveness means communicating what you want in a clear manner, respecting your own rights and feelings and the rights and feelings of others.

He needs to know clearly what you will and will not accept as treatment. "I would appreciate if you spoke to me with respect and kindness. That was rude and if you chose to speak to me that way then you will have to leave/then we will not continue this conversation until you can speak to me differently." He is 26 not 16 and he can and should leave if he can not behave in a respectful, kind manner. But you need to set the boundaries and expectations. And most importantly follow through with what you say. Or he will continue to be rude and treat you any way he pleases because you don't require that is change.

There is a lot of good information on the web and some great books on helping one become more assertive. That can really help in your situation. He is your son and you do love him dearly and just want a great relationship but sometimes we can make that happen so we are left with only the option of making sure we are, at least, not continuing to allow ourselves to be walked all over and treated disrespectfully.

I hope your relationship gets better.
p.s. Tell your husband (Dad) that yes it will do good for you two stand as a united front and to show your son that he will also not tolerate your son treating his mother that way. Silence is acceptance!