My cat keeps crapping on the couch

First let me tell you a bit about my cat. His name is Shadow and he's almost 6 years old. Normally has a good temperament and tends to be on the shy side at times. He's very loving and loves lots of attention. He's a bit nervous sometimes and tends to rip out his fur. But he seems to be getting better about that, so that's not so much of a concern anymore.
But in this past year he has taken a crap on the couch three times. The last time was back in April and before that it was in January. But today it was worse than it was those two previous times. I don't know what to do. Originally I thought maybe it was that his litter box was full, but that wasn't the case at all. I try to clean his litter box and change the litter as often as I can. Than I thought that maybe he couldn't get to the box, but that wasn't the case either.
My boyfriend thought it was maybe because I don't spend enough time with him. But I spend all my time with him when I'm home from work or when I have the day off. I let him sleep in bed with me and he has his own blanket. The only time I'm not with him is when I'm either at work or when I'm at my boyfriends. But when that happens, my parents spend time with him. It's very rare that he's alone. So I just don't know why he would be doing this. Please any advice would be very much appreciated. Thank you

Thank you for the advice.
He's a healthy cat. So I know it's behaviour. I'll try the litter. I hope that it works other wise I don't know what I'm going to do, if he does again.
Ro

even if you 'know' that he is a healthy cat he needs to be vet checked, most cats will use their litter box normally unless it isnt clean enough for them or there is an underlying medical issue. With mine I have to scoop the 5 boxes 4xs a day and change them every 1 to 2 weeks in order to keep regular box habits.