Just don't leave the sterile area (if you are checking a bag this calculus changes).

As long as you stay in the sterile area you can take the plane train down to the international gates and as Toshbaf says above you can physically get to the gate that the LH flight arrives into but you won't be able to meet your son there because he'll be directed immediately to the passport and customs/immigration section.

You can then proceed to the arrivals level and meet him.

If for some reason you are checking a bag, this won't be possible as you'll need to collect your bag in the Domestic baggage claim and then take the shuttle bus around to the International terminal which can take a fair amount of time.