You're very welcome, and thanks for the additional and helpful information!
I'm very glad to hear that your boy has always been very healthy! : )
When a cat begins to display the behavior you describe, it can be attributed to various factors. As you mentioned, he has companionship, so he's not crying because he's lonely at night; however, sometimes as cats age, their hearing begins to diminish and they will vocalize more frequently just to hear themselves and 'reassure' themselves that it's really their own voice. If his hearing is not what is once was, his own voice will sound different or strange to him, (that's why his recent cries may sound different to you), and the reason why he's going into the bathroom where it echos more and amplifies the sound. Because of this possibility, I'd have him checked out by his vet to evaluate his level of hearing and make sure there isn't something physically wrong.
Also, older cats can sometimes become disoriented (especially at night), and the increased meowing may be caused by the momentary confusion or panic of 'where am I, and how did I get into THIS room?'
He may feel unsure of how to get out of that room or unsure of where he wants to or should go, at that particular time. Cats can suffer from dementia or forms of senility, like humans. When he feels panicked by these incidents, he meows because he needs reassurance and security.
Do you keep nightlights on in the kitchen, bathroom, or other rooms of the house where he has to access his food/water, litterbox and/or the paths he has to take to get to them? If not, this would be a good idea. It's a fallacy that cats can see in complete darkness; they can see better than humans in low light, but do need a little light to see at night.
There are a few things you can do if he's feeling insecure due to loss of hearing or confusion at night. Try a product called Feliway. This is a cat appeasing pheromone which helps to calm stressed or anxious cats. It comes in a spray form and a plug in diffuser. The diffuser looks like an electric air freshener, but has no scent to humans. A few plug-ins around the house may instill a feeling of security and contentment in him while the house is quiet and everyone's sleeping. Feliway is available in major pet stores like Petsmart/Petco, etc., online, and some vet offices carry it. For more information, please see: www.catfaeries.com/feliway.html
As you can see at the Cat Faeries site, there are also flower 'essences'. There are some essences that also are found to be calming to cats. Click on 'essences' from the menu at the left on this site, to read more. Don't give him anything orally before discussing it with your vet, though.
Please let me know what the vet tells you, and if I can help further. Keep me updated on how your boy is doing. Thanks!