You will not know anything for sure, until you get an overall assessment for him, through Early Intervention.
GOOD, you have made that step to contact them.
Now just follow through, with it.
Your son's school, did alert you to their concerns, so that is good, better than them not, doing so.
But with any feedback or recommendations you get, you need to be realistic about it and do something about if, it is recommended for your son.
Because, getting this handled while he is still young, is important. And up until 3 years old, services are free.
When my son was younger, we got him speech therapy through our local Early Childhood Intervention program. They came, to our home, and it was free. And my son LOVED it and his Speech Therapist. It was, invaluable and even I, gained much knowledge from it. And even though, I personally do have, degrees in behavioral sciences.
Be open to, any feedback you get from the child professionals.
Don't look at it as something is "wrong" with my son. Look at it as, this is a GREAT opportunity for he AND you... to learn from and to help, him. Now, when he is still real young. And being the services are free, you are lucky.
I used to babysit a girl, what was very similar to your son. BUT... even if I alerted her parent's to my concerns and observations (which they did know about), they did NOTHING about it. And to this day, their daughter is developmentally... behind. And she is about 8 years old now. They too would say "its just her personality...." but this was, them being in denial.
AND, per their well-child check-ups, they did NOT tell, the Pediatrician about any concerns. They, avoided it. And that is pretty sad.
I would point blank ASK them, if they told their Pediatrician... and they said "no, she's fine...."
Also though, though children are typically very active, especially boys... instead of waiting for them to engage in something and waiting for them to play or do things... some kids need... to have someone initiate it. Not all kids are good at self-reliance and play. But also, he seems to avoid, running around. Again, so your son's school alerted you to this.
So again, follow through with the Early Intervention organization. Make an appointment. Your son does need to be there, and at least per my son's developmental assessment... it is NOT intimidating, it is NOT hard, and it is very child friendly and not "clinical" at all. The child just thinks they are playing. But my son got an overall... developmental assessment per motor skills and cognition, and coordination etc., even if it was primarily for his speech delay. And they give you the report on it. And discuss it with you.
With a young child... playing is, "exercise."
It is not about doing P.E. type classes or things.
It is just about being active. And if your son needs more of that, they will tell you and work with him, and you, on it.
And if your son needs services.... "therapy" is NOT like how it is with adults. With a young child, it is like "play."
And, being "athletic" or not... is not a requirement for children. Many kids and adults are not, athletic. So what. But the main thing is they are doing some physical activity... which is FUN... for them.
A kid, does not have to be, an "Athlete."
Kids do not have to do the soccer thing like all the other kids.
Main thing is, they do an activity, or play and run around with their parents.