I am sorry your boy is having trouble. Oh I would not lock him in the bathroom. I agree with you that it will only stress him further.
You do not need a bahaviorist for cats. They are very easy to understand.Everything they do has a reason.
Cats can have a UTI with no bacteria called Interstitial Cystitis:
Whenever a formerly clean kitty begins to soil out of box, he is usually trying to communicate some sort of pain or distress to his human "mom" or "Dad"
Cats may not seem unwell because it is their nature to hide an illness as long as possible but if the urine or stool is abnormal in any way - that will be your first clue. If his appetite is different that is an indication as well.
Your cat may have a urinary tract infection.That can become serious in males.
UTI's must be treated with an effective broad spectrum antibiotic like Clavamox or clidimycin and for a long enough duration so all bacteria is eradicated. If not, they will refluorish, stronger and more resistant and reinfect him. In addition, the bacteria can become tolerant of the same anibiotic so the medication should be varied for best results.
Anything new is very stressful to a cat because kitty's HATE change and stress can lead to illness so you want to watch for any developing symptoms.
Give him a teaspoon of plain yogurt every day. That will keep "good" bacteria in his colon.
He can also have one drop of unsweetened cranberry juice every day to lower the PH of his stool and urine.
If this continues or gets worse he should be checked.He is not just doing this to annoy you or because he got mad as cats do not think like that - he may not feel well and is trying to let you know about it.Or he is confused..
Sometimes cats become "picky" about the condition of their box and do not want to share or use the same one for both things..Try adding an additional one and see if that helps.
Also try cat attract litter sold in pet supply stores. Clean any soiled areas with an enzyme product so he can no longer smell it.Bleach will not fool a cats nose one bit.
He may be marking in a desperate attempt to reattain his head cat status or he may just be intimidated or sick.
Spray with feliway http://www.catfaeries.com/ to fool him into thinking items are already marked. Try rescue remedy sold in pet supplies and is rubbed on the gums.
Clean with an enzyme product like zero odor or odor mute because regular detergents will not fool him either.
Here are some additional links that may help (elimination and behavior related):
Please let me know how he is doing. if you still have questions, I will always be here for you.
I hope he stops real soon and he feels better!