I would suggest soaking it in a brine for 24 hours before putting it on the grill.  Last one I did I used a brine based on buttermilk, water and koshar salt.  On one that size I'd use about 2 cups of buttermilk and half a cup to a cup of salt.  Then I add some BBQ sause of my choice, a little ranch dressing some honey and some seasoned salt or other seasoning and add enough cold water to completely cover the meat in a big bowl or pan and leave it in the fridge.  When it's time to grill it I would rinse and dry the meat then rub some olive oil on it and the seasoning of your choice and throw it on the grill.

For pork seasoning I have been using some Cajun Injector brand Quick Shake Hickory Grill seasoning that's really good and also Lawlry's Perfect Blend Poutry and Pork seasoning is really good but I use it mostly for poultry and have found it's been to use a different seasoning on pork.

The brine will give you an extremely juicey meat when it's done cooking and even though it seems like it has alot of salt in it it won't make the meat taste salty.  The buttermilk is very acidic and will make the meat very tender, the last loin I used it on melted in your mouth like cotton candy.