As others have mentioned, It would be nice to know how much time you have taken to transition the food from one to the other. Did you do this gradually, or did you suddenly just change it? How much food are you feeding at one time?
Hopefully the stools are solid. Like Pat and others have said, its normal for a puppy, esp when very young to have to go potty right after eating. Of course this changes based on the time of day. In morning after being kenneled all night our new puppy(17 weeks now) goes directly to the front door and out to pee. We bring her in, she gets a drink of fresh water and eats breakfast. After breakfast its straight outside for a BM(Poop). This new puppy is always supervised when not in the kennel. We do not let her run amok unless we are watching her run amok and laughing. She is only allowed to run amok for our entertainment at this point.
We know from experience that after a puppy plays for 15 to 20 minutes, that they usually have to go out and pee. We make sure we do this also. When the new puppy is fed lunch, she will hold her poop for several hours. If I feed her at 11:00 AM, she will not usually go until 3:30 PM. I know this by paying attention. After we feed her dinner sometimes she does not have a BM until the next morning and will hold it through the night,
Of course every dog is different. The trick is learning each others schedules, getting them to fit one another, and paying attention and learning your dogs cues and routine. By the way our new puppy Lulu is a female. As I mentioned she is now about 17 weeks.