We don't know the nationality of your father, or whether he has any relevant residency permits, but it is more than likely he will indeed need a transit visa.
You can check whether he needs one using the UK government check if you need a visa website.
Transit visas, like any other visa, need to be applied for in advance. No visas are granted at the airport. Without a visa, your father is probably not even going to be allowed to board the flight to the UK.
Note that even in the event he were able to stay airside, depending on his nationality and permits, he may need a "direct airside transit visa" (DATV). This is different from some other countries where you never need a visa as long as you stay airside. Again, the visa check website will tell you so.
To get a more precise answer, please edit your question with details of:
- Your father's nationality.
- Any residency permits / long term visas he may hold, especially from countries like the US, Canada, Australia, New Zealand or European countries.
- Whether the two flights are on the same ticket/booking (single PNR).
- The airlines involved.
- Whether he will have checked luggage.