Can an attorney file his notice of appearance 2weeks after the hearing?

Filing a notice of appearance after a hearing does not "make the hearing invalid," nor does it make the attorney "not responsible" for it, or change the outcome in any way. Technically it may not be necessary after a hearing to file a notice at all - once an attorney appears in a case, the Court should note them as attorney of record in the file, and send further correspondence from their client to them. This is possibly somewhat sloppy practice by the lawyer, but it doesn't change anything substantive about the course of the case.

No, it does not. I agree with the comprehensive post by Attorney Bodzin.

See **** below: Had a hearing to change custody by non custodial parent. His attorney filed his notice of appearance after the hearing. Does that make the hearing invalid ****no or make that attorney just not responsible for**** no or is it not required to file such prior to represent a client and a case? ***** correct

If an attorney physically appeared at a hearing and verbally represented themselves to the court as being counsel for one of the parties, then that generally takes care of the necessary formalities for that given hearing. Filing a written notice sometime later is often a best practice, but it's primary purpose is to ensure that the court filing/docketing system has the attorney's representation status on the record, as well as the attorney's points of contact so as to ensure that they receive service of documents and the like during the course of the proceeding. Just because an attorney didn't file such a notice during the hearing is not going to mean that they cannot represent the individual, or that the hearing (and/or any resultant Order therefrom) is void or invalid.