Your son also sounds simular to mine. He is 3 years old and is high functioning autistic. He talks, not potty trained and has no desire to be, screams a lot, is social, but only to people he knows. He will say hi, bye and Thank You to people he dosen't know. He dose bang his head when angry, but is getting better with that.
If you son is 3 you need to get him in as many therapy's as possible to help him, they say the brain is wired at age 5, so you have 2 years to work with him. Have his siter is his life saver.
My daughter is 5 and she has helped her brother so much! You said your son will play with his sister, what about other kids? dose he mimick stuff, have sensory problems, is he more attached to one parent. Reseach High functioning autism and aspergers syndrome will will probably find it fits better.
Good Luck to you and keep in touch with us.  

TACA has a section for newly diagnosed parents. I remember ordering a free book from them, "first 100 days"...I believe that is the name

http://www.talkaboutcuringautism.org/index.htm

They are very good with newly diagnosed parents...check it out and hope this helps :-)  

well your son sounds like mine! except that he wasnt potty trained until he was almost 5. you will find that there are lots of levels of autism and your son probably is on the high functioning side. i dont know where you live but the best thing you can do for him is get help right away. if he is 3 then there should be a program for him at your public school. get him in as soon as possible. i have friends whose kids are now in a typical classroom and if no one told you, you would never know they had anything. my son is still in special ed (hes in 2nd grade) but he is coming along. there are days when hes doing great and others when well i just dont know what to do with him. and to make matters worse, his dad and i are divorcing after 14 years of marriage.
my sons speech was delayed as well and still is. his social skills are no where near his same age peers. the fact that your son has a sibling will do wonders for him. keep encouraging him to play with her as it will help his social skills and teach him to recognize social cues.
good luck. let me know how things are going.
(my son still hugs everyone! and he is 8)
jane