If he doing this even if he's just been out and even if he's alone for only 10 min, and going both pee and poo, it sounds to me like it may be more of a case of separation anxiety than truly having to potty. Whatever you do, don't ever punish or even so much as comment on it. There are lots of threads on separation anxiety, you may want to look up and read them.
Just very briefly, start very very slowly with him, take him out to potty, leave him only for *a minute* and come right back. When I say a minute, try not even that. Make your comings and goings VERY quiet, no dramatic "bye baby, see you when I get back" and then "omg, there's my baby, ooooh, (add baby talk and squeals here) ordeals. (not saying you do any of this - some people do) Just quiet leavings, and quiet returns. Literally practice, leaving only for several seconds, then come back. They truly need to learn ** over time ** that you'll come back. The peeing and pooping is out of nervousness (if it is separation anxiety) and they can't physically stop themselves. It can take quite a while and lots and lots of patience, lots of practice, lots of coming and going.
See if you can find old threads, I remember Fickla had an excellent thread a while back - or perhaps post more info here so we know what your routine is - I'll try to give you some ideas, you may be doing this already, not sure. Exercise him before you leave - take him for a good walk or play a good game of ball or whatever he likes (which means, especially for an anxious dog, at least a 30 min walk or a 30 min game of ball, not a 5 min potty walk, or 3 ball tosses). If he's tired, some of that anxious energy will be spent. Also, be sure you give your pup something great to chew while you're gone to occupy his time, maybe a frozen stuffed kong, a bully stick, something of that nature. You want to get his mind off his fears and troubles - and if you're only gone a few minutes, take that chew away as soon as you come home. You want him to look forward to getting it when you leave. You want that to be a *special something * he looks forward to, so that altho he may dread you leaving, on the other hand, he's got that incredible huge bully stick, or frozen peanut butter stuffed kong, or .... you get the idea, that he knows he'll get.
Am wondering also, how long have you had your boy? Has his schedule changed? He's still young, am wondering a few things here ... wish you'd post a little more info. What's his/your schedule?? Hmm....