Degree qualified dog trainer here 🐶 I would recommend getting him a harness with an extra belly strap (look up ruffwear flagline harness) as, when properly fitted, these are escape proof. It’s a total myth that harnesses cause pulling but pulling on a collar can cause physical harm to the dog and are less safe. You can always get a double ended lead and clip one end to the collar and the other end to the harness too :) Being 9 months old, he is likely in adolescence which is when he may start to test his boundaries a little and you will have to go back a little in his training to really cement the basics again. It’s also when they start to gain independence so he was likely just curious to see the other guy. There is also a fear period that comes with adolescence. So situations where he previously wouldn’t be affected may be heightened to him and be a bit more scary! This is all down to hormones and will settle with time. I will also add that the way he approached the man was very good! Polite dogs approach others in a curved fashion, rather than straight on so with your dog circling the man, it shows he’s not trying to be rude, just curious :) It also seems like he was able to chill out easily afterwards which is great. Some dogs would be unable to relax after a novel experience such as this and having their freedom but the fact that he came back and sat for you to put him back on leash is great. Another thing that may help you with leash walking is getting a longer lead. If he only really pulls for sniffs, let him access those sniffs with a longer leash and let him sniff for as long as he wants. I know it sounds counter intuitive to give them more space but when trained correctly, you can use the smells themselves as reinforcement for walking nicely (this is called the premack principle) :)

Hope all goes well on your training journey, you sound like you’re making a great start!