I transfered domains, but BTC transactions are pending now for 5+ days, he seems is not doing anything to fix that, because he was cheap and sent a 1000 and 1500 satoshis fee.
You did a wrong thing. You should never accept an unconfirmed transaction. There is no guarantee that an unconfirmed transaction will be finally confirmed.
If not will they be dropped from the pool in 21 days?
By default, transactions are dropped from the mempool, if they stay unconfirmed for 14 days, not 21 days.
Most nodes use the default setting and they will drop the transaction after 14 days, unless someone rebroadcast the transaction.