Is 24 and 31 a big age difference?

yes. in my opinion that is quite a bit of difference. but if you really like him go for it.

I am a guy,& it does not matter to me if my gf is about 6-7 years younger than me.
Anyway its better to u to say it true,as one or other day the truth will itself come in front of him.

I was in the same boat.
When i started working, i was bricking myself thinking i wouldn't be able to date anyone, because there would be a big age gap with people in different departments or outside work. And i wouldn't be able to converse with them because the age difference would mean different experience in a different time, and both physically and mentally.

But i discovered that it makes no difference whatsoever. I date women who are a few years younger than me, because their maturity is on par with my own, and it is so gratifying dating someone younger due to their ability to make life so beautiful, spontaneous, fun and interesting. This friend a girl 25; she was dating a 39 year old, after 6 months they are engaged (mouth dropped on the floor when she told me).

I think it should be fine if you tell him. If not, he then hasn't developed that maturity yet, and doesn't even deserve your company.