Autism or not?

Okay I'm going crazy, my sisters 4 year old child is displaying (to me anyway) signs of concern.  From the beginning he never hit any of his speech/motor milestones.  He did learn to walk at 18 months, and learned to talk around 2.5-3.  She put him in therapy for both.  According to her drs said he had a speech delay and problems with the muscles in his legs.  Physcially now he seems totally normal & can run, jump etc with a normal gait.  As far as speaking goes he talks like a 2 yr old.  Usually choosing to grunt or use gestures instead of sentences.  If je wants spmething he will say "milk" or "snack" rather then "im hungry can i have a snack" etc.  He will always nod/shake head rather than say yes/no.  He is capable of saying sentences but doesn't very often & especially not around other people in his core family unit.  At preschool he does NOT talk & does not have friends, but does do activities alongside the other children.  He is no longer in therapy (not sure why) as i believe my sister believes that because he CAN talk everything is now okay.  Also, there is a phrase he learned from tv- "let go yo yo" that he says ALL the time in inappropriate circumstances.  For example if you ask him his name he might say "let go yo yo" & then laugh. It seems to be a filler for him when he doesn't know the answer or is too shy to speak the answer..?  Sometimes he ll just run around singing it- sometimes he ll chant it over & over.  The family always found it amusing, but recently I read about delayed echolalia & am now concerned.  Another oddity is in his play.  He plays with his 2 year old brother & usually their play - conversation involves saying some word (ex yo yo or butterfly) over & over.  The 2 year old repeats it & laughs & then the 4 year old repeats it & laughs etc.  Never do you hear meaningful words from the 4 year old (like "your turn, or here it is etc).  Lastly I should add that this little boy is very affectionate.  He will hug extended family he has met once for more than 1 min.  He seems to want to be around people, although after a few min of playing might go to the other side of the room to do his own thing.  He loves stories, puzzles, play doh & coloring.  He doesn't like loud noises (public restroom toliet flushing) & is very particular about his food (ie no crust on sandwich, no discoloration on a cracker etc).  He makes some eye contact but usually looks down in a bashful way, as if too shy to hold it.  He does respond to his name by looking up.  
Could it be he is just shy?????