The only DNA test which is acceptable is one done with the court approved facility where there is no possibility of forging the result. So his result is worthless if it was one of the DIY results.
Unfortunately, they are expensive and the last time I looked at the cost it was about GBP800.
What you could do of course is a DIY test yourself where you know what happened with it and that he didn’t cheat.
If he refuses, the presumption is that he is the father.
If you refuse the presumption is that he is not.
Unless you have multiple partners, then you would be in a position to know whether he was the father or not.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have