Praise be to Allah.
If the interruption was lengthy, such as an hour or two, then he must repeat tawaf and start all over again. But if the interruption was brief, there is nothing wrong with that. That is because in tawaf and sa‘i, it is stipulated that the actions be consecutive, meaning that the circuits or laps come one after another. So if they are interrupted by a lengthy break, the first circuits are rendered invalid, and he must start tawaf all over again. But if the interruption is not lengthy, such as if he sat down for two or three minutes, then completed it, there is nothing wrong with that.
Majmoo‘ Fataawa Ibn ‘Uthaymeen (22/293).