Re: is this sign of autism
I don't see that this is autism for sure as this can also be confused with just being 15 months. But, what is important is to go to your son's doctor and tell him what your concerns are. They may make a referral for you to get a further evaluation with a developmental pediatrician, or your son's dr. might make a referral for early intervention services.
The important thing is to get early intervention going soon as there is a great deal to be gained from this.
The office for developmental disabilities in your city can give you information about early intervention services too. Your local school district special education office is another resource for early intervention. The good news is that there are lots of services and therapies nowadays and your child is young and can benefit greatly!
But I would take these concerns to the pediatrician and they think it's the right direction, make the referrals for early intervention. From what you have described I cannot honestly see definite autism but I do know that the early intervention is a good route to go.