My 4 year old won't talk and no one knows what to do

My husband and I adopted our son when he was only about 6 months old. He was developing like a normal baby, babbling, rolling over, he hit all the milestones, even his first words and everything. Then a little less than a year ago he just stopped talking. He'd always been a quiet kid and his preschool teach always mentioned how she wished he'd talk more to the other kids in his class. Our routine hasn't changed, we'd been contemplating adopting another child but hadn't told him yet and now we don't want to until we get this sorted out. He's a great kid and always very courteous and listens better than most adults, but for whatever reason, he just doesn't talk. We've been taking him to a speech therapist and this last meeting he basically told us he didn't know what was wrong and maybe our kid's just mute. I don't know if that's possible since he used to babble and talk before this. He will randomly say something to me or his father, but no one else outside of our house and never more than a handful of words, and if he can get the message through without talking, he will. Has anyone else ever experienced this before or have any ideas of things we could try? What are the chances he'll just grow out of it?(I'm at work right now so replies may be a bit short in supply for a little bit) via /r/Parenting http://ift.tt/2GLwvNc