Dog becoming aggressive

I need some advice. My very large dog has been slowly becoming aggressive towards strangers over the last few months, even children. It started a while back when a child asked to pet him and I said sure, he’s always been friendly so why not? But he growled at her. Today another child approached him at the park and before they could even get within 10 feet he started very aggressively barking. Why is this happening? He’s 2 years old and not fixed, will it help to get him neutered? Is it because I’m pregnant and he’s more protective? I’ll add that he’s never EVER shown agression towards me, my kids or my husband... he’s extremely loving and protective of us. It makes me so sad that my beautiful dog is acting this way towards people he doesn’t know 😭 any advice?