please help

My son is 41/2 years old, he has mood swings through out the day, he has trouble sitting for a period of time and at times he has to be asked a couple of times to do something. He also throws bad tantrums. He also shows no interest in writing letters or doing school work. He is great with his baby sister and is very loving he is extremely attached to me. His father and I are separated however he sees his father regularly. I am currently with someone else who lives with us and is the father of my daughter. My son is mostly around adults, since birth has never really interacted with other children because there were never any around. He is fine when playing with another child one on one but hates crowds he gets very upset when there are allot of children around like at a birthday party. I am very concerned about him because in preschool his teacher says that he has no interest in making friends and allot of the time he does not answer her questions or participate. However, is not like that at all at home. I don't know if his behavior is due all of the changes that are going on around him or if it is something else. When his father and I were together we faught a lot and sometimes he saw the fighting. I am very worried that something serious might be wrong. Please help. Thank you.

[margypops]

These are regular behavioral  problems I dont think they are anything worse.For a start he is probably jealous of his little sister, and is acting out, and will get better in time,he hates crowds but then a lot of people do,how about he has old fashioned shyness and doesnt want to mix doesnt mean he has anything major wrong with him ,give him time and pay him a lot of attention he may be feeling left out. Can Dad do more things with him and give him some time for Guy stuff.