One of my sons is hypotonic, which means he has a neuromuscular problem that leads to delays in gross and fine motor skills. At some point when we had him at the polyclinic for a check-up, I was given a diagnostic chart that I still have here. This is what they expect an 18 month old to be able to do:
Gross Motor and Movement: Climb up onto a chair. 75% can walk backwards. Walk while pulling a toy. Walk steadily. Bend down and pick things up.
Social Reactions: Take off shoes and socks. Look at picture books. Roll a ball with an adult. Drink from a cup. Go and get something for an adult. Imitate housework.
Speech and Language: Identify pictures. Identify at least one body part, e.g. “Where’s your nose?” Uses 6 to 10 words; passive vocabulary larger than active (i.e. the child understands more words than s/he uses).
Fine Motor/Eye-Hand Coordination: Scribble. Build a tower with two blocks. Throw a ball.
Object Relationships and Problem Solving: Pull a toy closer using paper. (That is, if you put a toy on a piece of paper, with the toy just out of reach of the child, s/he should figure out that pulling on the paper will bring the toy closer, too.) Put one shape in a three hole shape sorter. Put rings on a ring stacker and take them off again.
Now, this is the minimum - most kids can do more than this in some areas at least. If a child is unable to do all of the things in one area, there’s a chance of some developmental problems and intervention should be considered. If he or she can’t do any of the items in two or more areas, then it’s definitely time to look more deeply into what the problem might be.
From what you’ve said, it sounds like gypsykid is lagging behind in gross motor skills, but there’s an explanation for that. It does not sound like he’s significantly behind in speech. There’s a huge variation in normal speech among toddlers, so the gap between “slower than the neighbors’ kids” and “time to start panicking” is enormous.
If it makes you feel better, by all means talk with the doctor. But in the meantime, try to relax, tune out your mother, and trust that your little one is developing at the pace that’s right for him.