Advise on getting a Uk passport for my son

[007cableguy]

Hi

Not sure whats the best to do ,

I'm English married and have a 6 month old son to a Thai , I'm about to apply for for a UK passport for him and there are a couple of things not sure on ,

Firstly I'm renting a condo would a copy of rental agreement be ok for proof of address or would it be worth getting a residence letter from immigration ?

Secondly I'm going back to the uk in a couple of weeks for a couple of months would it be possible to apply for one over there if I have the correct paperwork?

Thanks in advance