I am very sorry for your loss of his brother.
Cats grieve just like people so he probably gets a little lonely at night.
He seeks you out because YOU are now his safety net.
I am so sorry you are having this and the howling problem.Hyperthyroid can be the cause but so can hearing loss.
Older cats experience confusion but it is usually related to hearing or other sensory loss.When an older cat begins howling at night it is almost always associated with hearing loss. The cat is not totally deaf but his hearing is reduced and his own voice sounds so DIFFERENT to him he will go around howling so he can test the tonal quality of his voice much like hearing impaired humans speak loudly or nasal.
There are several things you can do. Play a radio softly at night so he does not "hear" so much quiet. Leave a soft light on so he does not "see" so much darkness. Get some feliway plug ins as these will calm him down, he can get very anxious at night. You can get them at some pet supplies or http://www.catfaeries.com/
Older cats suffer from other diminished senses as well. Loss of sight, smell and hearing can cause confusion and disorientation. This is quite disturbing to your baby Kitty dementia is quite common at this age too.
There are vet scripted drugs like anapril that can be very helpful for dementia and sleep aids as well but.you want to try homeopaths first, get rescue remedy at a pet supply or products at:
Feed him a canned food meal with turkey as it contains l-triptophan and is sleep inducing.
The fact that your baby is alive and well at his age is a tribute to you and the remarkable care you have given him .May God bless both of you.
Try these suggestions and you can enjoy his old age in peace - and quiet.
Please let me know if you still have questions. I will always be here for you and your baby
Warmest best wishes,