dont drink 1500 cals at once.. Thats not gonna help
like ez_e said you need to slowly and steadily increase calories at every meal.. Through out the day.
Ok so you said you are not home a lot.. You need to invest in a really good cooler you can carry around w/ you and pack it with good food.
I leave my house at 645am and dont get home til 9pm most days. I eat like a fiend regardless. I have at least 4 meals and 3 snacks in there plus a shaker bottle and protein powder.
There are no good reasons why you dont eat enough.. Just excuses.. You have to overcome it and find a way that works for you to pack in those calories.
I used to stash a ton of food at my office.. Now i just have boxes of protein bars
i used to carry around a bottle of olive oil. Added it to all my shakes and just added it to every meal.. My cholesterol was super good and my joints felt great.. I also packed on a ton of weight!
You can do it you just need to work at it and figure out a way to make it happen.
If you have questions keep asking.
Good luck.