So I have a couple of questions regarding my puppy and I was wondering if anyone could give me advice on what I should do

1 He has some runny poop followed by what looked like a little bit of constipation that is no longer a problem as his poop is solid and normal. His food was switched so was that possibly why?

2 He’s been itching a lot. I checked him thoroughly before bringing him into my house as I have other dogs and washed him up with blue dawn a couple times. I thought this was because he didn’t like his collar but he does this even when it isn’t on him. Could this potentially be due to an allergy that could be solved by grain free food or something else? (He will be going to the vet in a couple weeks to get his next shots)

3 He also has a gap in the side of mouth, I know he’s still young (15 weeks) and teething but shouldn’t he have all of his teeth by now?

And lastly any food suggestions that aren’t blue buffalo?

Thanks in advance