I have a jack russel terrior and he is not very vicous but when my grandson comes round who is 2 he gets very aggressive and goes to bite him, I get worried when my grandson goes near his face and strokes him. What should I do? I have tried mussles but they don't work.

What do you mean you have tried 'mussels'?

Muzzle perhaps?

Don't understand why the correct size muzzle won't work in your situation. Pleas explain further.

An improperly fitted muzzle is of no benefit to anyone, or is it because he reacts badly to the muzzle? Ie: flailing, gator rolling, pawing wildly at it, etc.
In any event, it seems that having him wear a muzzle throughout the visit vs. allowing him to stay in a place that he is happy and comfortable, like his kennel or his favorite resting spot in the house with that door closed off seems a little harsh.
Is your grandchild rough with him at all, or ever pulled at his tail or legs?
Two year olds, as well as JRTs can be pretty rambunctious, your dog may simply not be comfortable with the amount of activity there is during his visits.
Couple that with him possibly feeling that the child is an intruder in his territory and you have a pretty upset little dog.
I'm all for correcting the behavior, but when it comes to the safety of a child, common sense is your best friend.
Is your grandson there often or only occasionally?
What do you mean when you say " not very vicious" ?
Is he normally a little snippy?

My experience with jack russels is, they get bored really easy, too much energy, too smart, not enough exercise, walk him more, keep him active.

I was think of crustaceons when I read 'mussels', when the OP meant to type muzzles (on a jack russel,give me a break).

You need to let him know you are the ALPHA (boss if you are not familiar with that term) and stop this behavior instantly. He is ruling your household and that is not acceptable.

LadySam, jack russels are snippy, teeth bared when they are unhappy with a situation, so I see this happening here. The two year old is taking his place in attention is what is going on.

If your dog is acting aggressively towards a child, than keep the dog away from the child. If that means that you need to put your dog in a different room completely during the entire stay of the child, than so be it.

Some dogs do not like babies. There is nothing you can do in the world to make them like the baby. Babies smell different.. they don't smell normal.

If you cannot isolate your dog, than I would suggest to rehome the dog. Talk to breed specific rescues and tell them his problems. Breed specific rescues can place dogs into homes that have no threat of children (or anything else the dog doesn't like).

Obedience with a behaviorist is always a good decision. See if you can even work with this dog over this issue.. However; even if you get the issues under control, the dog and the child must never, EVER be alone with one another.

Muzzles are a good tool. However; I wouldn't use a mesh muzzle. Purchase a cage muzzle (the greyhound muzzle). This isn't 100% though with a young child because the child will still stick his/her fingers into the muzzle openings.

I still think the best solution is to remove the dog completely from the situation. Would you like to be around someone you cannot stand? I know I wouldn't.