Have you considered having an emergency pack of Naloxone as a just in case measure? It can be lifesaving but if you need to use it, remember that it will not be appreciated at all at the time.
Has he considered trying any therapies like maintenance prescribing (methadone or subutex) or even to tone down the cravings like auricular acupuncture?
Although if he is using several times a day he has a very strong habit and isn't wanting to give up as he is being enabled to do what he wants and I'm willing to guess that daily living tasks like paying bills, rent, food etc are not the first priority when he gets some money, it's always the drugs.
Are you using protection for your own sake if you are having sex as I have known users to trade their body for drugs which then having unprotected sex with you puts you at risk as you in turn are having sex with possibly many others unaware of the risks to yourself.
Have you considered going to therapy for yourself as you need support to deal with all of this, you can't save him from himself, only he can make that choice but you are enabling him and that's not healthy for you. Also as a partner you will need support in your own right, remember in his mind that the drugs come first.
Remember if it comes down to it and a choice between your life on the line and drugs, 99% sure the answer is always drugs. The answer to everything is drugs.
The feeling of getting one over on the support worker, does he have any medical conditions as I can guess that any medical appointments come down to him trying to see what he can get. This is very familiar.
I have a ex friend (platonic) who I have known all my life, drugs is the first thing in his mind no matter what the situation.
His partner who when she was admitted to hospital and was being given morphine by the Dr, he distracted the Dr very briefly who looked over his shoulder and in that moment he stole the syringe which was just under the partners skin but not injected yet and ran locking himself in the bathroom and getting a hit. That was her breaking point as it was the drugs, always the drugs before anything else.
I know you might not take any of this on board but protect yourself and remember you can only ever be second to drugs. I wish you luck.