Don't worry about the surgery changing his personality. He's not going to change one bit. I totally understand your worry. I've had many cats from my early childhood on. Many of them boys. Right now I have 5 cats: 2 3 year olds, and 3 8 month olds. Leo and Sonic (8 months, but huge) have stayed just as affectionate and didn't change at all after their surgery. Neither did Hero, or the girls.
In fact it actually helps to keep them happy and affectionate and relaxed. When they go through the hormonal upheaval every so often, it can cause frustration in them and stress.
Thankfully, the surgery for the boys does not take long and they receive pain meds and I've never seen any of them in pain afterwards, or even upset. Usually, they start playing and being silly as soon as they are able.
I hope this helps, and if you have any questions, let me know. Have a lovely Caturday with your sweet Benito! Purrs from my 5 cats!
RE: My beloved Benito is all grown up, he is really adorable.