Help potty trained at home but can't do outside

[Lwek]

My son is 2 and 9 months and is potty trained at home. He stands up weeing and poos on the toilet. We started potty training 12 weeks ago. If we go out I have to use pull ups and take him to toilets but not intrested. Have tried without pull ups but he just wees and poos in pants and will even walk round with the poo in pants and not be bothered what do I do help ....

What happens when he needs to go at home? Does he ask?

Yes he asks sometimes and the majority of the time he goes and takes himself to the toilet he only shouts when he needs help.