My son was born in the UK.. we are having to come to Thailand by a tourist Visa, I will get a job as a teacher and obtain a working visa, am I able to just add him to my visa as a dependent? Or do we need to get a non immigrant O visa?

There is no working visa. There is a work permit and a non-b visa or extensions of stay based upon working.

Before your child can get a dependent extension of stay they will need a non-o visa. You can get it at a nearby embassy or consulate after you have your extension of stay.

Thank you! So he can't get a non O visa until I have an extension of stay on my non immigrant b visa? That will take 3 months so how can my son stay legally in thailand until that point? Will I need to get a few tourist visas for him until I can extend and get him the non O?

Also do we have to leave the country to get him the non O or can I just do it at the local immigration office?

You might be able to get one when you get your non-b but normally they will not do them until you have your extension.

A single entry tourist visa would him 90 days total by getting the 30 day extension.

Children under the age of 15 do not pay overstay fines so if was not able to get a visa it would not be that big a problem.

Edit: Immigration do not do conversions to a non immigrant visa for dependent extensions.