Buyer delays closing date

[Marci Stein]

Contract states buyer must close on or about 45 days , which is 3/24 /20. 
I’m told he’s waiting for mortgage approval , although it’s been over 6 weeks and he was pré approved .

I’m dealing with a seasoned investor who most likely is delaying till warm weather , and I’m stuck paying taxes . can I cancel the contract , on grounds the date has come and gone? 

I know everything is either closed or delayed now , so I don’t want to be insensitive, but I do believe buyer is taking advantage since closing can be done by mail . 

Thanks!