“The son does not accept my new man – what should I do?”

Alexandra, hello. In fact, you are asking a rather difficult question, it is not surprising that a psychologist in kindergarten did not give you specific advice.
Usually children of this age quite calmly accept “new” parents, and such problems are gradually solved with the efforts of adults. But since in your case, these efforts are being made, and the child does not make contact, it may be worth paying attention to the relationship between you and your man.
I can say that despite all the assurances that you will love him no less, the son may not react, because young children do not perceive words the way adults do. First of all, they “read” emotions and feelings, and not the words of adults.
Therefore, in this case, you can analyze your relationship with Andrey. Are you personally satisfied with everything about them, are you satisfied with them, do you feel safe with this man, do you consider him the “ideal of a man” that your son should strive for? Have you become even a little happier since this man appeared in your life or plunged into even greater worries? If it turns out that something does not suit you, then you can discuss it with your man and find a way out of the situation, you can go to a psychologist and work out your fears about the future with him.
In fact, as practice shows, when a child has a tense relationship with the chosen one of his mother, very often this is only a reflection of some unspoken problem between the adults themselves, which they should discuss with each other and solve it among themselves. If you do this and Dima’s relationship with Andrey improves, then I think you will get what you wanted, if not, then you still have to contact a psychologist.