is there a suitable head harness suitable for a boxer that will prevent him from picking up everything while we are on our walks. i know there are some on the market but i dont think they will be suitable.?
The head harnesses like Gentle Leader and Halti are tools to keep the dog from pulliing only. They don't restrict the dog from open his mouth so they will do nothing to keep him from picking up things. You might be thinking of a basket muzzle. This fits over the face and straps behind the ears and has a metal or hard plastic "basket" that goes over the muzzle. It will prevent the dog from biting things but not restrict them from drinking, or panting, or even licking through the spaces in the basket. Make sure you fit your boxer correctly--most basket muzzles are made to accomodate a dog with a longer muzzle. There are brands that make basket muzzles to fit boxers and other short nosed dogs. Whatever you do, don't use a fabric muzzle. These are meant for temporary restraint only and fit too snug and keep the mouth shut tight--not good for using while exercising for any dog.
Of course my first recommendation would be to train a good "leave it". Additionally, a strong recall or call to heel will help if your dog is off leash and approaching an area where he is likely to forage. Do a search on "leave it" or "recall" and you will find a number of good posts that outline methods for training these behaviors.