Thanks for the additional information. It is helpful.
Lamb and rice are pretty common food ingredients. I'd do one more food switch to a unique protein that the dog hasn't had before such as limitd ingredient venison and sweet potato. This should help if it is an allergy. However, he will not be able to have any other foods or treats. Instead of canned pumpkin, try metamucil just on the off chance pumpkin is affecting him. I actually don't feel food is the issue but it won't hurt to switch and see. Read more on allergy conditions here:
Often with allergy conditons, a dog will have ear infections, a poor coat, and hair loss in addition to gastrointestinal issues. Cyclic instances of diarrhea like you describe are often the result of protozoan infections like giardia and coccidia. Now the problem with protozoan infections is that they are spread by cysts in the stool that are only shed intermittently so they dont' always show up in stool. They are spread in the stool. Thus all stool in the yard has to be cleaned up as soon after being deposited. In addition, rain washes the cysts into standing water like puddles. The dog sheds them and later drinks from the puddles and becomes reinfected again even if treatment is provided. So all feces need to be cleaned and standing water cleared as well.
Now it sounds like your vet tried treating for giardia with the metronidazole but if the fecal matter was in the yard or there was standing water, he could become reinfected. Coccidia is treated with a different antibiotic so talk to your vet about retreatment and possibly treating for coccidia as well. Here are some sites on those infections.
Another thing that should be mentioned is that irritable bowel syndrome (IBS) is often cyclic as well with reoccurance every few weeks. Read more on this including various treatment options that some try on the following site. Now this site has multiple pages so be sure to look over them all by clicking on the various pages.
Most adult male labs are between 65-85 pounds. At 6 months of age, I would expect him to be about 50 pounds but if he is from working stock, he might be a little less than that. Feeding him a highly digestible food would also be advised and having less filler in the food would let you feed less food by volume but might allow him to gain a little weight since there wouldn't be as much filler. He should also produce less stool as well.
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