after prostate surgery urn flow very weak

What should I do and is that normal for a 71 year old man

Welcome, Smitty.
Have you called your urologist/surgeon's office and told them what's going on? There are many possibilities, including obstruction and swelling of the urethra if the catheter was removed just recently.
If you think you aren't voiding your bladder completely and are retaining urine, you must let your uro/surgeon know, that's a medical situation that needs to be addressed right away.
Also, please give us some more details: when was your surgery? when was the catheter removed? what was your stream like before surgery? has your stream been very weak ever since the catheter was removed, or is this a new development.