It's true as saxy_jag says that children often put speech development on hold while learning to walk and vice versa. Had your step grandchild been a 'babbler' earlier on I would have said it's probably nothing - some children choose to skip the one-word "sentence" phase altogether and go straight to two-word-sentences around the age of two. But the fact that your grandchild didn't babble is cause to take action now. You're not being overly concerned.
It needn't be anything 'bad' but he'll still benefit from an evaluation and ideally also a referral to a speech- and language pathologist, who will be able to make an assessment of what kind of communication is best suited to help him get his meaning across. Many children who are late speakers benefit immensely from learning to use simple gestures. This is a scary thought to many parents, who tend to think that "if we all use gestures and that works out really well, why would he ever bother to speak?!" but in fact it's just the opposite. "Sign language" aids speech development, and anyone who is at all able to develop spoken language will always do that in the end. Learning to use gestures can speed the process up greatly and above all make life even more joyful for him and his family right now.
But that's just one scenario out of many possible ones. My point is, no don't feel silly for being concerned and yes do act. If he does need help, this is the right time. If he turns out to be "just late", you still won't have made a fool of yourself, no one will think so. Be sure to mention the non-babbling bit. Good luck, and good on you for bringing it up.