Olga's answer is the wrong way round: she has interpreted the defining clause as a non-defining one, and vice versa. Jacque Cilliers, meanwhile, seems to have missed the point about defining and non-defining clauses entirely. The only correct answer you have been given so far is in Kelley's comment above. As Kelley has explained: 1. His son who is a doctor came to see him this afternoon . This is an example of a defining relative clause. The words 'who is a doctor' cannot be removed from the sentence without changing its meaning: this is why there are no commas. The subject of this sentence is "His son who is a doctor". The fact of his being a doctor is an essential piece of information, as this fact distinguishes this particular son from his other son or sons. The man in question has two or more sons, only one of whom is a doctor. The next sentences might be something like this : "His other two sons rarely visit him. They are both sailors and they spend most of their time abroad.". 2. His son, who is a doctor, came to see him this afternoon . This is an example of a non-defining relative clause. The words 'who is a doctor' appear between commas: this is because this information can be omitted from the sentence without changing its essential meaning. In this sentence, the son is not defined by the fact that he is a doctor: this is merely additional information. The implication is that the man only has this one son. I hope this helps.

Both sentences are correct, but their meaning is a bit different. From the second sentence we can state that he has more sons, and the information is about that one who is a doctor. The first sentence just tells us about his son, where the information regarding him being a doctor is additional and less important.

We use commas like brackets. So your sentence would look like this: His son (who is a doctor) came to see him this afternoon. Both sentences work just fine, but I would use the one with commas, because it breaks up the sentence well and makes it clear that his son is a Doctor.

@Kelley : Thank you so much for helping me!

The first one means he may have more than one son. In the first one, "who is a doctor" can not be removed (therefore no commas)It is necessary to tell the reader which son -the one who is a doctor.. In the second one, because there are commas, it is just extra information and can removed. It is just acting as an adjective. There is only one son (and he is a doctor).