Anytime your boxer has really loose stools (diarrhea), he needs to see a vet. Many if not most canine diseases are capable of causing diarrhea in an affected boxer, so you need to know what condition your boxer has to treat it accordingly. Also, if your boxer has eaten something that has not agreed with him, or if he has been poisoned, diarrhea is likely to result. The cause should be determined for your boxer's safety and well-being. Any condition causing severe diarrhea can be fatal if not addressed, especially in young puppies.
If you are a gardener, you need to be aware that cocoa mulch is toxic to dogs in much the same way as chocolate, as are hanging baskets that are made of cocoa mulch. If you use this type of mulch, make sure that it is in an area where your boxer has no access. Don't use it at all if you can't keep your boxer away from it.
If your dog is suffering from diarrhea, it could mean that he has ingested some kind of poison. The most common way that dogs get poisoned is by eating rat or gopher bait. If you suspect that your boxer has eaten some of this poison, or eaten a poisoned gopher or rat, you should get him to your vet's office as quickly as possible. Most common rat and gopher poisons rely on anti-coagulants to kill the rodents. These are toxic and often fatal to dogs as well. So if you suspect your boxer of being poisoned, your vet needs to know in order to administer the correct antidote. Be aware, too, that snail and slug baits are poisonous to dogs and usually very attractive to them.
A lesser known but equally potent form of poison for many dogs is any form of diet pill. Herbal diet formulas that contain ephedra and guarana can be equally lethal. The canine metabolism works quite differently from the human metabolism, and diet pills can affect your boxer's heart, kidneys, liver, and brain.
Another common source of poisoning for most dogs is chocolate. Chocolate contains a compound called theobromine, which is toxic to dogs in general. However, the toxicity for any particular dog is very hard to predict. Some dogs (including some boxers) have eaten large amounts of chocolate with no ill effects, yet others die from just a few bites. In general, the darker the chocolate, the greater the risk of poisoning to dogs.