If you have pain when you need to have a bowel movement or if a stiff BM causes pain as it passes past the prostate, then you will have pain when a penis, a finger, a dildo or another object presses against the inflamed prostate.
Different people react differently depending on how the prostatitis is affecting them and what kind of prostatitis they have. There are four basic kinds, with two chronic variations, one being bacterial the other non-bacterial. Non-bacterial is the most common one.
http://www.mayoclinic.com/health/prostat...n=symptoms
I had - past tense - bacterial prostatitis when I was younger and was in the role of 'bottom' in bed. For me it was too painful to accept my partner 'up there'. It was also painful to ejaculate - misery in all ways. Every morning as I was getting ready for a BM I had more pain. Even having a BM was a 'thrilling' experience.:o
I would suggest 'probing' gently with the second finger of your own hand. Wear a latex glove. if you are allergic to latex then get a non-allergenic glove. You can buy them by the box at most drug stores.
Use a water based lube like KY-Jelly - again sold in drug stores. Gloves are found in First Aid aisle, lube is found near the condoms which is usually on feminine health products aisle.
If you are laying on your back, insert the finger pressing down (toward your back, not toward your testicles/penis). Then slowly turn the finger and press gently upward where the prostate is.
You should pretty much know how sensitive you are with a finger and can judge how painful it will be with a large object.
The problem with using a finger is that there are bones in your finger. A man's penis is just soft tissue with blood swelling it up. Thus a penis is 'softer' and more flexible than a finger. A dildo, even the floppy rubber ones are not as soft and 'giving' as a penis thus I would not start off with even a slender dildo.
I would go back to the doctor and discuss your particular case. Ask him/her about anal sex and prostate massaging. As others have pointed out some people respond well to prostate massaging, others not so much and even may have complications.
On prostate Massage/Drainage:
http://www.chronicprostatitis.com/massage.html
This last link has illustrations to give you an idea where the prostate is and this will allow you to probe yourself. The prostate is not that deep inside, thus is easily found.
Read that site first and do read and remember the warnings.