Is there is a legal way for me to access dads account, since he is no longer capable of managing it himself?

I don't see any need to contact social security since his check automatically goes into his checking account.

Your POA gives you the authority to access his checking and other accounts for the purpose of managing his money and paying his bills. Take a copy of the POA document with you to the bank for them to have. 

As far as his taxes go, they are filled out on the same forms like anyone else's, signed by you as his POA and you may want to send a copy of the POA in with the return. If you don't want to do his taxes, then have a CPA do them.

Spoke to SSA directly; they said if I bring a doctor's letter to a local SSA office I can get access. His neurologist wrote me a letter and I will see if that works.

Unless things have changed very recently - Social Security does not recognize Power of Attorney.

Take your POA (financial permission) in person to social security office and ask for tax information document for past yr. That may be quickest.

You could call SS and see if they will just mail tax info again to your dads address if that is still valid.

Lastly, since you are POA, you might consider forwarding his mail to your address so that you can get a handle on his finances, bills, etc. you can do that and set a start and end date.

Go online to socialsecurity dot gov and create an account for yor Dad. The POA is sufficient and there is an option to get a duplicae tax statement. You can view it online and print it so you would have it immediately.