Another possible reason for speech delay is if the child is growing up in a multilingual household. As you haven't mentioned this I assume that there is only one language spoken in your house, but in case anyone else is having this issue it may be relevant to them to know this (and of course, not all multilingual children have speech delays). A woman I knew spoke both Hungarian and English to her son. Until about three he understood both languages, but spoke neither. At three and a half she knew he'd have to start pre-school within a few short months, and that he needed to be able to talk in time. So she decided to drop Hungarian and use only English instead. By the time he went to pre-school he was talking very normally (unfortunately he has forgotten Hungarian!).
When I was around 10 my English teacher told our class that she had had a speech delay - she said not a word until she was around two and a half. It's more common than you'd think.
The important thing is to make sure you speak to your child regularly, even if they don't respond. Point things out, describe them, get them interested. Face to face talking is apparently best. Talk to your doctor as I can see you are deeply concerned, but don't jump to conclusions - and stop googling autism symptoms! It is feeding your fears, it will not help you. Losing weight over such a short period of time is not healthy, and there is probably nothing to worry about.