Dr-Exchanger is one of the oldest exchanger of bitcointalk I know
If he is saying he have $50k dollars worth of btc then he have. He is the man of words
Stop asking for a signed message from such members. Go ask newbies
This members where as well legit till they scammed :
Master-p
escrow.ms
Muhamed zakir (or something)
and the list can be longer!
So it's totaly fair to ask him to sign his wallet