Recognizing Red Flags for Autism

I've been talking to our pediatrician and we are concerned about some of the red flags we see in C5.  At 3 1/2 he is hyper-good at puzzles  (More than 100 pieces).  He also likes doing the same puzzle over and over again, and he even seems to have an order he likes to work the puzzle in.  He lines up his Hotwheels and Matchbox cars.  He has also lost some of his foods he will eat.  He gets melts down a lot--especially when he sees foods he doesn't like.  He has texture issues with his hands and clothing, and I see what I think are stimming behaviors.   

C5 also does mostly parallel play, and I see him have a hard time with imaginative play. 

We've decided to get him evaluated at the same medical facility who diagnosed his older brother, C3, as having ADHD.  This center is attached to a nationally known hospital and they have a division that works specifically with children and their growing brains.  

I want to determine if he is indeed on the autism spectrum like I suspect he is, and what I might do to help him so he doesn't fall behind.