Make him wear boxing gloves. :-))

You need to explain to him that he is only allowed to pick anything with your permission. otherwise you are likely to find him picking plants/flowers that are poisonous.

i found saying NO with a reason why did it for me and mine.

Tell him its only you that picks the ripe fruit or veg, unless you tell him that anytime you are picking different items in the garden that he may help you.

Otherwise tell him he is on allowed to do it when he asks your permission , and you are with him .This worked with my 2 and they are now 19 years old and 21 years old.They now dont touch nothing unless they are cutting down weeds or mowing the lawn

be more calm and assertive

You need to be firm, and expalin why he should not do it. But it all comes down to 'NO' meaning 'No' . Our 5 soon learned that they would be allowed, indeed welcome, to eat from the garden when they were given permission. Our 7 grandchildren have learned the same lesson now. Believe me, when my wife says 'NO' she means 'NO!' and nobody, not even me, dares go against her.

I was very firm with my children they were only allowed to pick things from the garden when they knew its name and 3 things about it. This worked for us on the whole as they couldnt be bothered unless it was strawberries and peas!!!

I have the same problem with the dog. I made the mistake of giving him the odd raspberry, blackberry, and even cherries when I was picking them and eating them myself, and he soon picked up the idea he could get them off the bushes himself.
He has me well trained now, and expects the choicest berries from the bush. Our other dog has been seen leaping into the air to pluck plums off the low hanging branches of the plum tree.
You couldn't make it up!

im afraid thats your fault bertie but you dont seam over botherd . my mum used to have a clever alsation that once in a while would nudge the fruit bowl for an apple and get one as long as she didnt take advantage and she didnt. maybe once a month .