Go back to basics, begin again as though you've just got him and he's a wild bird. Follow Michael's taming guide:
http://trainedparrot.com/Taming/. Start by having him out of the cage for a really short time, start with five minutes - whatever is the amount of time you can keep his attention through training
without any biting. This is really important -
you need to stop the biting - and if you can only handle him briefly at first then this is what you need to do. You need a month or two of this regime to break the biting habit, start with a short amount of time and build up
very slowly, a minute at a time. I really recommend you follow Michael's guide, particularly the target training. All birds seem to love this and it is the way forward for you. Target him on and off your shoulder, target him out of the cage and back in. Use a hand held perch at first if that will stop him biting your fingers. The whole time you have him out keep him occupied doing things that he enjoys, such as trick training, flight recall, target training. Don't give him time to get bored or restless, as soon as you feel his attention is wandering, put him back in his cage. Give him lots of praise and treats, you need to rebuild trust on both sides. I'm telling you this because I've been through something similar with Alfie, my Sennie, following an illness during which I had to give him antibiotics twice a day for five days. After this he started biting and it steadily escalated to the point that he was biting me every day, then started biting my face. I was frightened of him and was at my wits end. I started following the above regime (after consultation with Michael) and the situation started to turn around immediately. Several weeks in and our relationship has improved 100% and he hasn't bitten me once (....well,
once, when I answered the phone while he was out of the cage...I knew better than to do that

). I feel as though I'm back in charge, and my sweet, affectionate little gremlin has returned.
I hope this helps. Good luck with it - you
can turn this around and enjoy the relationship you'd like to have with Marvin.
You might think about getting Michael's book too - I think it covers a lot of this kind of stuff and his methods work