It is high time he (change) his diet for a healthier lifestyle.

• It is time/ It is high time এরপর যদি subject থাকে তাহলে verb টি past tense এ হয়।
- It is high time we left the place.
- It is high time we stopped corruption.
- It is high time he changed his bad habits.

Complete sentence: It is high time he changed his diet for a healthier lifestyle.

অন্যদিকে,
• It is time/It is high time এরপর যদি subject না থাকে তবে to + verb হয়।
Example: It is high time to stop corruption.

Source: Applied English Grammar and Composition, P.C. DAS.