yes I feed him 2 times a day and I have hay out there but he dosnt touch it and its been raining everyday towards 3pm and then it gets soaked and I don't have a barn to put him in to make him eat the hay
Until you're the vet sees him and is able to advise you on a weight gain diet, I'd start with the diet I suggested. You don't need to weigh it every time, as you'll learn how much equals what weight with experience, but it's a must to begin with. Remember, this is a temporary start until your vet is able to guide you in a specific direction, diet-wise!
I also agree with MakeYourMark. As someone who lives in a very sandy area, I can vouch for the importance of psyllium! Tractor Supply carries SandClear and DuMor SandAid. It will be important to feed him the suggested amount for one week out of the month!
Do you know when he was last dewormed? From the information you've provided, it sounds as if he hasn't been vaccinated yet this year, which is something you'll need to talk with your vet about!
As far as feeding to gain weight goes, senior feed is just fine! I have a lot of personal experience bringing extremely underweight horses to healthy weights, Mudpie included! Senior feed is your friend. ;)
Here he is about a month after we started to get him proper nutrition:
Here he is about two years later:
Right now he's on stall rest and on a diet, so I'll spare you the pictures of his chunky belly and we'll just focus on in-shape Mudpie ;D