There is one thing you didn't mention in your question. Whether your son likes this group of people. It's easier to join a coterie if you admire and genuinely like its members.
If his main reason for wanting to be included in the "clique" or "gang" is loneliness, he risks appearing desperate and, even worse, sycophantic. Any attempt to break the barrier will be thwarted.
On the other hand, if his main concern is to be part of an efficient and productive team, then it is fundamental he finds out who the leader is. In any typical group, there is always a leader, it's usually the person who has been there the longest and has earned their peers' respect.
Once your son has identified who that "leader" is, he can approach this person and ask them for advice. Not advice on how to be accepted, but advice about work, patients, doctors, etc. that sort of thing. Even if your son already knows his job, and exactly how to perform it, he still needs to show respect (and some deference) to his colleagues. There is always something new to learn. See also @Charlie's very valid piece of advice
Instead, the focus of the conversation should be on his coworkers, and how things operate from a social perspective, on his new floor.
With luck, the "leader" should be able to impart invaluable tips and advice about the floor which your son can put into immediate practice.
It takes time before a group accepts a stranger in its midsts but if he is humble, kind and willing to put in the extra hours at work (for a reasonably limited amount of time) it will get easier.
To help oil the wheels of bonding, he ought to invite colleagues for an evening drink, an aperitif or arrange to meet at the local coffee shop. The first time he should offer to pay, he's the "new guy", and he did the inviting. These small social gatherings help people to get to know each other better as they tend to be friendlier when they are relaxed and outside work.
Asking if colleagues have spouses and children is always a good conversation opener, and from there you can ask about their kids' ages and if they have any photos of them they would like to share.
Next time, he should head confidently to the table in the cafeteria where everyone is sitting and ask:
Would you mind if I join you? It's just for five minutes, I have this niggling problem with blah, blah, blah. (make it work oriented)
Who will be so anti-social as to reject a polite request? By stating it will only be five minutes, he is acknowledging that they may have better use of their free time than to discuss his problem, it is their break too, but he is also saying it will not take up all their break time.
Show his co-workers that he works hard and does everything he is expected to, and more. Never be late for work, and forget about asking favours, e.g. “Can you stand in for me next Monday morning? I need to pick up mom at the airport” However, he must show a willingness to do favors and go that extra mile. If he does the above and displays common courtesies he will eventually earn their trust and respect.