i will be travelling to the UK with my GF and our 2 year old son in couple of weeks staying for 12 weeks.

i am a UK Resident so I don't need Travel Insurance to cover Medical costs but I will be taking a Policy out for my GF, will I need Travel Insurance for our son, he has a UK Passport?

If your son is resident in the UK then he, like you, is eligible for free access to the NHS, if he's resident in Thailand then, in theory, he isn't.
I say in theory because I don't think a NHS Trust would check his residency status in the event he needed follow up treatment, emergency treatment is free to everyone, but they might and they could possibly check if he's registered with a GP.
If I were in your position, given your girlfriend is already taking out insurance, I would be adding your son to the policy, the extra cost probably wouldn't be a lot and it covers more than access to the NHS.

It's always worth taking out travel insurance when going from one country to another as things can go wrong. My wife came here last year to live and a 12 month travel policy with BUPA for £80. Absolute peanuts for the benefits of the policy.