It's usually possible to ask a child of that age what's going on. You'll probably at least get a sense of where he is emotionally on the topic. Which might give you ideas about how best to support his needs. You may be able to convince him he'd do better using the potty, but this is really an area of development that must ultimately be in the child's control.
My guess is that he's still on the early end of success for boys, and he's realized that he's taken on a life-long "obligation," at which point many littles regress. When they are truly ready for training, both physically and emotionally, they'll pretty much go at it until they succeed, with the same persistence that they had for learning to walk or talk. And when that time arrives, training happens very quickly.
Yes, he's physically capable, but the emotional component is huge. And his accidents could have undermined his confidence that he's really up to the challenge. If he's laughing now, it could be sheer relief that he's let go of that anxiety. That he has taken control of a situation that may have been somewhat ominous, with it's ever-present potential for failure.
You could, at this point, make it a struggle, or you could leave it up to him to choose diapers for now, give it a few more weeks or months, and wait for him to decide he'd rather be trained, after all. You'll have a whole lot less cleanup to do, and he'll have the opportunity to come at this developmental milestone when he's really ready to take on the ongoing commitment. free of stress. Once he's convinced you won't try to pressure him into a task he doesn't want to do, or feels uncertain he can do, he may revert fairly soon to being a big boy like his brother.
