Sudden Tax Payments needed?

My father received in the mail a notice for him to pay 2 installments on September 15 and December 15 for the 2026 tax year even though he just paid his 2025 taxes earlier this year.

He's retired and lives off his Canada Pension and is really worried about if he doesn't pay that they'll just cut him off and then he won't be able to pay anything.

I've looked through the options and seen that one of them is to do the calculations for 2026 but it's not even done so either way it feels like he's just getting screwed over.

If anyone has any assistance of help it would be greatly appreciated, it sucks seeing him get panicked out of nowhere when he would normally just pay his taxes for 2026 in 2027.