We have this as well ... We stay firm, especially if we have asked her who she wants then changes her mind, as we then explain she chose which (parent)...it's awful hearing them cry but this is age that we need to stay firm, stick to boundaries and rules unless they are unsafe in anyway...
We tend to validyhis feelings but reinforce who is doing it. "I know you want mummy to help, daddy is there, daddy can help you right now, can you please let daddy help and then we can.... " For example if getting shoes on, we'd then say, then we can go in the garden/shopping/etc ..
You're not alone. We usually just try to ignore it or say that "mommy can't do it now but I can help if you want" sometimes it works, sometimes it doesn't. Just hoping it is a phase and he will get over it but he likes to test our patience 🫠