Since your JRT is still a puppy this problem will probably be fairly easy to control with some consistant training.

Have you and your dog been trough training courses yet? I reccomend you do some basic obedience with him. It will help you two bond and it will also just help general things and make him no that your alfa.

The leave it command is also crucial for your puppy to know. There are various ways of teaching this command but the way my previous dog learned it very simple. Put your dog on a leash and drop a treat infront of it but just out of the dogs reach. When you drop the treat say "leave it". The dog will try to lunge at it and get the treat but do not give in and give it to him. When the dog calms down and looks at you say "yes" and give him a treat. Do this alot, repetition is good. When you think you have it down pat, switch the treat up and see if it is still consistant.

Eventually you will be able to take your dogs on walks and say leave it and they will stop sniffing/eating.

Do NOT use a muzzle. A problem like this can easily be fixed and a muzzle is not solving the problem at all. It is just putting a band aid over the situation.