Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.
Again I do apologize that my colleagues could not aid you sooner. If you would still like assistance, can you tell me:
How long has he been doing this?
Do you mean that he is sitting with his front lowered and back legs standing (example
Any tenderness or tensing if you press on his belly?
Any appetite loss, vomiting, or odd signs otherwise?
Can you take a photo of this? If you can do so, this will let me see what you are seeing. To post them online, you can use the paperclip icon on the tool bar above your text box. Or if you can't see that on your computer/phone, then you can post them on any other site (ie Flickr, Photobucket, imgur, etc) and paste the web link here for me to check.