Hi K.- you haven't given a lot of information here, so I am going to give you some general advice based on my experience as a divorced mom ( my son was 4 when I got divorced) who is about to get remarried to a great guy who my son loves.
1)You say he is mean and 'disrespectful' to your boyfriend when you are not around. Have you ever actually witnessed this or is this just what your bf is telling you is happening?
You don't say how old your son is, but it is perfectly normal for him to be suspicious and even jealous of your boyfriend. YOU are the whole world to your son- even more so after a divorce. Being forced to share you with a strange man can be a very scary thing for a kid.
You don't say anything here about your boyfriend, his situation, how he tries to relate to your son. This is VERY IMPORTANT. He is not your son's father. He deserves t be respected - but trust and respect are a two way street. PLEASE be very careful about who you go out with and how they treat your child!
Is your boyfriend kind to your son? Patient with him? Is he a parent himself? Does he understand the pressures and responsibilities of being the 'grown up''? Does he understand that it may take a long time for your son to trust him and that is perfectly natural?
Too many women get divorced and then start going out with a guy who gratifies THEM, without looking at how that person treats their child. If you say your son is always respectful with other adults, do you know for sure he is being rude to your boyfriend when you aren't there, or is that just what the boyfriend is telling you.
It's great to get a new boyfriend, don't get me wrong. But the primary relationship here is between YOU and your son. You owe that to the child you brought into the world. Leaving his father was a hard decision and I would never question that it was the right one. But don't let the fun of a new boyfriend distract you from what is more important- your son and how the two of you are doing together.
Sit down and talk seriously with your son. Explain to him that you like your boyfriend and have fun with him and ask if your son feels the same. Reassure him that you love him BEST, no matter what, and that no other person can come between your special relationship as mother and son.Explain to him that even if he has reasons for not liking your boyfriend, it is important to be polite and respectful to adults, him included.
Ask if your son feels uncomfortable or unhappy or even scared of your boyfriend and LISTEN to what he says. He must trust you to be honest with you.
I don't know what kind of guy your boyfriend is. But too many women just get involved with some guy who has unrealistic expectations of their kids. If your kid was just be a brat in general, that would be one thing. But he has very good reasons to be nervous with your boyfriend and that can turn into back-talk, etc. But be VERY CAREFUL about what your boyfriend terms 'disrespect'. Ask yourself what he has done to win your son's respect =- and yours!
If this situation puts too much pressure on your son, it may not be the best time to be in a new relationship. Seriously, your son has to come first here. A really good guy will wait if he has to, be more patient, change his expectations and behavior. Talk to your son about this issue and LISTEN to what he has to say about your boyfriend. Good luck!