So you're 120 in years so your grandson is 120 months, there's 12 months in a year .so times that by 10 - he's 10 years old. How many weeks are there in 10 years? 52 weeks in 1 year so times by 10 again and theres 520. So your grandson is 520 days old, 365 days in a year. 520 - 365 is 155. So he's 1 year and 155 days. The average month has 30.4 days per month (365.25/12) So 155 days would be 5.1 months or 5 months and 3 days.
So the final answer is 1 year, 5 months and 3 days.