Autism or not?

My sisters son is 4.  He did not walk until 18 mo (dr at the time said it has to do with weak muscles in his legs, braces corrected the problem), he did not talk until 2.5-3ish.  I think at 2.5 he had 3 or 4 words.  He did go to speech therapy.  My sister was told he was a perfectionist & wouldn't speak until he could say the words with perfection.  The therapist said he has a speech delay & by the time he started school he would be completely caught up.
He is capable of saying sentences "I don't like this food" or "I want to watch tv" etc but normally just uses one word like "snack" or "tv.". Most of the time he'll point & shake his head/nod VS saying yes/no.
He is a big fan of a local commercial & says a phrase from it constantly "let go yoyo.". He's been saying this phrase daily, sometimes multiple times a day ever since he could talk.  He uses it inappropriately, like if were watching a show & I ask him who his favorite character is, he might answer, "let go yoyo.". Or if you ask him his name he might say that.  It used to seem he said it out of silliness, bit now it seems to be a crutch- like he doesn't know the answer, or perhaps cannot answer, he ll just say the phrase. I have read about delayed echolalia, however he just uses this one phrase... I don't hear other ones... But then again I'm not around him all the time.
He appears to be very shy.  If you ask him a question he ll usually look down (in a bashful) way and not answer.  
He recently started preschool & is not talking (at all to anyone) there.  It's been months of no talking.  My sister thinks he's very shy.  

[ConcordCA]

He doesn't have friends but will play along side the other children.He is a social boy, he seems excited by the idea of playing with cousins an he plays with his 2 year old sibling on a regular basis.  During play he makes up nonsensical words (baaahbop bop bop) and shouts them and then laughs.  Never do you hear many "real" words like "your turn" etc.  recently I noticed him going off by himself during group play activities.  He will come up to people he barely knows and Kim of lay across them in these sort of awkwardly long hugs.  Is it possible he is just shy?? He sleeps well & doesn't have terrible tantrums