Sorry Boeing Europe for the weak attempt at humour above...I realise it's a serious predicament for you.
I have no personal knowledge of SkyCrewing but did spend a winter season on secondment in Bangkok with a reputable operator during which time I saw the possibility that there could be some shady types loitering at the margins...and some ropey old operators.
I notice that the individuals name behind this appears on Linkedin: maybe that's where you spotted the job ad. If you are registered you can perhaps find out more about him and his past. I am not so I haven't. I guess you could also then canvas a couple of people on his contacts list. He may or may not have a reputation (good or bad) in that part of the world. Would also be interesting to know what is being written on those Thai cabin crew forums which google up!
Presumably he hasn't told you the name of the operator but if it's 737 cargo only ops in Thailand it may not be too difficult to work out who it is and confirm with them if it's a genuine position.
At some point before contract signing he will have to reveal his and his Clients credentials to you including proper contact details, and I am pleased that there is no suggestion you are being asked to hand over money or bank details (just copies of all your personal papers). Maybe you can insist on a face to face signing of any contract and see if he becomes evasive; tell him you are popping in on your way back to europe and see if the threat of door-stepping flushes him out!!