Hi, A.!
From what I can gather, not walking by 13 months is still well within the normal range for kid development. My daughter started walking around 9 months, but her contemporary, who's only a day older, didn't really start 'til 13-15 months.
I definitely think it's nothing to worry about, but I want to mention one thing. Our close friend's son started scooting on his butt and was still not walking (or really even standing or cruising) at all by 18 months. We were concerned about this because we could see he'd developed scooting as a habit, but our friends didn't want to think anything was abnormal. Unfortunately, their pediatrician didn't caution them at all prior to that point but then strongly cautioned them once he saw the boy at his 18 month appointment... He continued to scoot until nearly 2 years old and had to have physical therapy to strengthen his legs and unlearn that easy mode of transport. He's fine now, still a little weaker, maybe, but he's making progress and keeps up with other kids his age just fine (He's not quite 3).
So, what I'd say is DEFINITELY don't stress out! But do make sure your son is getting up and putting some weight on those little legs so he gets used to being upright and liking it. They have terrific stationary bouncers now (some people are afraid of the ones that hang from the door jam -- we weren't, but we know others are). That could definitely add fun and strenghtening for little legs.
As for the talking... that's another thing to not worry about, I'd say. Our daughter didn't say much 'til around 15 months, and by 18 months had about a 300+ word vocab! Now she never STOPS talking and speaks in paragraphs of properly formulated sentences. I think, with kids, it's progress that matters, not necessarily particular milestones at prescribed intervals. You know if he's comprehending -- so just keep challenging him and he'll rise to the challenge, I'm sure :)!