I'm guessing he had no idea about baby's age or about BLW, and possibly thought you were going to force feed a two month old with a T bone steak! But still. He didn't need to be so rude about it! It would've been hard to have a sensible comeback to him at the time, but a letter of complaint might educate him and make you feel better. Can I ask if the butchers was in North or South Bristol, as we are also living in the city and regularly use our local butchers.
And my children have been camping and to BBQs all their lives. There's always plenty for them to munch on and much more fun eatin outdoors. Of course you'd be sensible and keep them away from the cooking area, same as in the house.
Try not to let one person's ignorance get to you. You really do know best