Likewise with Dougal. He was starting to mark around age one.
We had him neutered VERY soon after that He will be 2 in October and is still not 100% fully housetrained. But at worst he will have one wee during the day if we're at work. Usually we're back by half 3 and he can hold it. He's completely reliable when we're at home.
One thing though that I've found is SO important is to continue treating them and praising them when they go outside. Dougal goes on command straight away every time he goes out, and gets praise and a treat. He gets extremely excited about this. However it's taken lots of hard work to get to this point. A year ago I'm quite sure he was still having accidents while we were at home because he had cottoned on to the go outside = reward.
When you say he was outside for 20 mins, was he just put out and left for 20mins, or did you stand out with him and wait for him to go? Because if he doesn't know that when he's put out its for toilet he will probably spend the 20mins trying to get back in to you, too distracted to go to the toilet, and then will go when he comes in . Dougal's housetraining took a bit of a step back when we stopped being totally consistant with him. Once I started going out with him every time and treating him as soon as he went things got back on track.
Best of luck
Cecily, owned by Dougal (B&T boy, age 2) and Dora (Blenheim female, age 2, rescue)
Cavaliers at the bridge, much missed and not forgotten: Aggie (tricolour female) and Rio (Blenheim female) and Tandie (ruby female)