Get Your Dog Care Questions Answered by Experts
Some people think that pets eat strange things because their diet is lacking important nutrients. That's probably why they recommended the vitamins. They are not a bad idea. Try to find a vitamin that has omega 3 fatty acids. This will help to give him a healthy coat and will make for a healthy dog.
When he has an allergic reaction, you can give 25 mg of benadryl by mouth over 12 hours, no more than 3 doses.
I suggest that you remove all the weeds. Cut them close to the ground with a weed eater and put houseplants where he cannot reach them. If you see him go for the weeds or plants, distract him with a toy or treats.
Let me know if you still have questions.
The glands could be because of an infection or problem that needs attention. It could also be part of the allergic reaction.
Excess water consumption can be an indicator of diabetes and sugar imbalances, among other things.
It would be impossible to give you an internet diagnosis at this point.
I suggest that you remove the allergen and see if it helps with the water and glandular problems that you are having.
When in doubt, see your vet!