You don't necessarily have to change food brands. If Vinnie is ok on Skinners and you are happy with it then you can just move him onto their adult variety when he's old enough. I change mine over to adult food when they're around 6 months but others leave it longer - anything up to a year, it's really up to you. If you want to change then do a search on here, there are LOADS of threads on various foods which members use and recommend.
The amount you feed will depend on his weight and the brand of food you are feeding - there are always guidelines on the side of the bag which you should use as a starting point, you can then 'tweak' the exact quantities if necessary. I generally feed my dogs by eye, I know how much they should get in each feed and if they put on weight I adjust it down and if they lose weight then I adjust it up. As they are working dogs I adjust their feeding in the winter to allow for their greatly increased workload.
