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My son had a sever reaction to the first MMR jab. Does this mean that he is well protected from these diseases, or is a second dose still necessary?

If a child has responded to all the components of the vaccine the first time, he will not have a problem being exposed to the viruses again. It's like any one of us who is already immune meeting someone with the disease - the infection can't get established.

If he hasn't made protection to all three diseases after the first time, then he would still be susceptible to those natural infections, and still needs the 2nd dose.

Reactions after the 2nd dose are essentially the same as after the 1st dose, but if they do occur they are even rarer. There are no new side effects after the 2nd dose that do not occur after the 1st dose. The advice is therefore that it is safe for your child to have the 2nd dose in order that he is properly protected.