It would be best to check with the rescue to find out what dry food, veg & hay the bun has been on. The packaging for excel adult pellets, says from 16 weeks, i.e. 4 months of age. When you get your bun, the rescue should either give you some of the food he is on, or let you know which type so you can buy a small packet of it, and slowly change over to excel adult pellets ( I usually change over by reducing old food and adding new food at a rate of 10% a day, but some buns with sensitive tums may need a more gradual change).
edited to say that at 7 months, he should already have unlimited hay...some buns can be fussy with hay! Again ask the rescue which hay he prefers and see if you can get the same hay before trying new types.