Concerned about Spartans diet! Or am I just worrying too much?

OK the script is that since Friday Spartan hasn't done anything but drink water and take treats when I reward him for doing the right thing. I got some of the same wet puppy food he was eating but he's not interested in it whatsoever.
I initially thought at first he'll come around eventually and that he's just stressed a bit in his new environment and is off his food! Anyway about an hour ago he got another reward for going to the toilet in the proper place for a poo.(He really likes to spread his pee around if I turn my back on him!LOL! ) Then I got to thinking what if.....? Because he will eat something!
I already had a bag of Wainrights puppy food waiting to slowly swap him over from the wet after giving him a bit of settling in time and thought what the hell i've got nothing to loose so I opened the bag and mixed some in.NOTHING!! I binned the wet stuff and then gave it him pure dry softened up with some water.
Well as soon as he got a whiff of it he turned into a mental berserker mini rotweiler and nearly tore my knuckles off to get to it. So that confirms it! All that time Spartan's thinking to himself, 'Sorry pal but there's no way that your gonna make me eat that CRAP any longer!!'

All I ask from you in advice is that I am concerned that the food has been straight swapped and not mixed over gradually because as soon as he sniffs the other stuff in the mix he just leaves it alone! Is there any extra precautions I should take going down this extreme path to get him to eat or is it ok to continue on knowing he's going to eat happily?

Ideally it would be better to introduce new foods gradually. I'm not an expert and others may not agree with me, but I think with a pup as young as Spartan I would be more concerned with the fact he is not eating and if he is not eating the wet food at all but is happy to have the dry food ( either softened or as it is.....assuming it is a specific puppy formulation ) I would give him that.
His may change initially due to the sudden change in diet but hopefully sort itself out soon after. Hopefully his belly will handle the change ok.
This is just my personal opinion.

I'm sure others will be along soon to add comments

i'm with the advice above, better for him to eat something rather than nothing.

dont worry about worrying you'll do loads of that at first, i was a nightmare worrying bout every little thing!

As above, really. It's always better to gradually change over, but a lot of dogs will accept a change without problems. Just be prepared for a few runny poos over the next day or two!

Thanks all, he does seem to be a lot happier now and his poo today is actually a bit more solid from when I first got him.
Also now everything he does round the house is in Turbo Charged MODE!!!

As long as you have found something he will eat it should be okay. His stools may be a bit runny for the next few days. Normally you want to mix a new food in gradually but they tend to have strong stomachs so it may be okay. We are still looking for the right food for Suki given her allergies and she swaps over without any issues.