Trying to look at this situation from your son's perspective.
The brother(s) who study with him have probably implanted the idea of baptism into his head under the guise that it is a dedication to God. What loyal, emotionally charged young Christian wouldn't want to show God how much he/she loved Him by making this public dedication? He also gets to sit in the special place at the convention with thousands of eyes on him and enjoy all the accolades that come with it. Pretty enticing.
How are you going to combat that without coming across as a God-hater... a position that will only confirm the warnings his "teachers" have probably already given him about you, thereby strengthening his resolve?
That's a tough situation.
I can think of three approaches off the top of my head:
1. Lead with making your son KNOW that you approve of his love/dedication/sincerity for God.
2. Calmly show him how Watchtower has perverted baptism into a loyalty contract with the organization and how such a thing is not scripturally supported.
3. Show him how even Jesus - THE SON OF GOD - in whose footsteps we are supposed to walk, waited until he was VERY mature before making such an important decision. Then ask him to contemplate the reasons why Watchtower would want to hurry him into such a contract. (Keeping in mind what you said in your OP about how the elders will expect your son to treat his own father after baptism)
YOUR CALM, LEVEL HEADED APPROACH WILL BE VITAL.
Just spewing what jumped into my head. Hope this comes across as at least semi-coherent.