Visitor visa change of circumstance

hello, my partner has a six month Visitor visa. When we applied we said he would just be staying for about one month as he had to return to continue his studies. However now that has been postponed due to some issues with the university so my question is:

can he stay longer without notifying UKVI, or do we have to formally tell them about a change of circumstances? would it make a difference if he stayed one more month compared to another three months, for example?

other circumstances have stayed the same, and I can prove enough salary and savings to support him.

thanks for your help!

sorry - to be clearer than my previous message: I assume he will technically be allowed to stay for as much of the six months as he chooses. But we plan to apply for a 5 year route spouse visa soon, later this year, so I am worried it will count against us if the information we gave when applying for the visitor's visa changed and he stays longer than he said he would.

also, will it seem as though he is living here rather than visiting if he stays for longer? He was in the UK for three weeks in May for the birth of our son - how do they calculate the cumulative time spent here, is it by calendar year?

He may stay in the UK until the expiry of his leave as a visitor. No need to inform UKV&I.