I think the difference between an unkowns 3bb standard raise and a 3.5 raise is indifferent at this point. If hes a real donk maybe hes hoping the smaller raise will get him more action, or maybe its a drawing hand trying to keep value down. I think the difference is real miniscule and without other information we can assign him a std utg raise for 6 max. Anyone like 22+ AT+ KQ+ and QJs+. I think that sounds about accurate.

I think the real question is what does his check-raise here mean? After a lot of thought I still think you have to felt it here. The flush is just too strong to lay down to an unkown. Now if he is proven to be a real solid player I think I have to consider folding.