Weight gain problems any advice?

I need advice on how to gain weight i am 26 years old 6ft 5inches tall and have the build of a skeleton about 10 stone. It doesn't seem to matter how much i eat i just cant seem to put on any weight i am getting married in October and my partner will look absolutely amazing but i feel im going to look horrible in all our pictures etc even though my partner says i look fine. i can see my ribs when i lie down and its really starting to get to me any advice would be appreciated. i read on a site that i would need to consume around 3000 calories a day to gain weight and that target just seems impossible also i have a very active job and im on the go from start to Finnish for 9 hours a day which will obviously burn calories. hope i have provided enough information.