Hmmm, humping is not abnormal and not usually sexual with neutered dogs. It does seem a little odd that he doesn't seem to know he is doing it and is embarrassed. Poor guy. Maybe it's subconsciously soothing to him. I'm reaching, I honestly don't know.
Welcome to YT. I'm sure someone smarter than I am will come along.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226