It seems that we had a problem with the chat feature.
I would first look at your relationship with your husband. Has he been in the habit of lying to you about things? If not then I wouldn't expect that he's lying about masturbating. I would speak to his doctor about it, because some medications can impact semen amounts. If you're relationship has been good aside from the bi polar, and he's not been in the habit of lying, I wouldn't jump to the conclusion that it's something he's doing.
I welcome your thoughts, let me know if you want to talk more