We are here also (but 5years on with an almost 10yr old).
They will have an idea of expected height based on you/ohs height to determine what is believed to be normal height.
They will track his growth for a good 6-12 months. This determines if he is small but growing or just not growing enough.
Ask for a hand X-ray to confirm bone age - this determines the growth plate size and longer term plans.
They will likely send you to endocrine team for hormone checks (growth hormone test isn't nice but worth doing)
Once they have all this info plus a full history, they can work out a plan to monitor, support, enhance etc. This will include things like OT referral to support size, guidance for school etc.
For us, we have low insulin growth factor but growth hormones are ok. Has a genetic disorder which is known to cause small stature so is agreed this is the cause. And aim is for supporting him and his X-ray suggests he has a 2 yr delay on growth so will hopefully keep growing 2 years past peers.
Get all the info, be prepared to accept small stature as a disability that may make things challenging for your son. Build up a resting b*h face for when people make comments on his height.
At home, make sure it's accessible as he gets more independent (can he reach light switches, the toilet, get steps for the kitchen etc) so he isn't held back by it.
Feel free to message me directly, it can be a lonely place.