Corso and socializing as an adult

I have a 4 year old cane corso, I got him a few months ago. He came to me with a solid obedience training. (Heel, break, out, stay etc..)

Overall, he’s a good boy. I don’t expect him to be friendly with strangers because I’m not sure how well socialized he was and also, he is a guard dog after all.

But he really doesn’t like people coming to close to me, he will start to get suspicious and might growl.

When strangers come to our home, he does growls at them. He is usually in a down stay in place just to make out guest feel more at ease. But if someone tries to pet him, he will not allow it. Usually growling stops once I say “no”.

My question is, can adult dogs be socialized? And if so what’s the best way to do it with a cane corso?

I have a frenchie, and I got him as a puppy and he extremely well socialized. He was touched by many people, he was interacting with many dogs so I for sure see how important socialization is