Regular SMS won't send to only one contact?

So I've had my iPhone 5s for a few months now, and had no problems until now. One of my contacts will try and send me a text from his android, but it won't actually send, and vise versa. It's really quite frustrating, as neither of us have blocked the other, or anything of the sort. I've tried restarting my phone, resetting the network settings, deleting our conversation and just deleting and re-adding him as a contact, all to no avail. Can send and receive with all other contacts. Please help.