International tax advisor/accountant/attorney needed - US/Germany

Hi everyone.  

My client is looking to live in the US... but work for (and obviously get paid by) a German business.   He will work remotely via computer in the US for about half of the month, then travel to Germany and work there for the other half of the month.

Apparently it may be a tricky, tax situation... so looking for an expert on this matter.

Any referrals or guidance would be appreciated.   Thanks.