You could always design a t-shirt for you:

'My dog does not like you stroking him
I do not like you stroking him.
If you stroke him, he might not bite you
but I certainly will '

You see, any sign on a dog presupposes that onlookers can (a) read and (b) understand simple instructions.
After many years of sending notes home for parents I have come to the conclusion that this is probably asking too much.

thank you all i will look at the options.

Right, first of all you have prior knowledge that your dog is not comfortable in certain situations so therefore your first responsibility is to ensure that he is never put in a position where he puts others at risk; you, as his owner, have a duty of care not only to him (and your dog does not actually have to BITE a person to be pursued under the DDA); in addition, you have a duty of care to OTHERS who may be affected by your actions or inactions.

The hierarchy of controls in ANY situation, is first to eliminate risk and only THEN, if you cannot, put in other measures.

It is a no brainer really, as there can be NO possible justification for NOT muzzling your dog. You cannot control OTHERS but you CAN control your dog. Failure to do so not only leaves you liable to criminal prosecution but also to a civil suit due to negligence.

Is it worth taking risks with a) your dog's life b) the life of others and c) do you have money to burn?

Putting on a collar or coat with a specific warning on, will not materially reduce the risk to:

illiterate people,
people who do not have English as a first language,
children too young to read,
the visually impaired
Dyslexic etc etc etc.
mentally handicapped

So, teach your dog to accept a muzzle happily and maintain his safety and that of others. A basket muzzle is the most practical as a dog can drink, pant and even have treats.

HTH