What are some signs that a man has lost interest in me over text messaging?

Read the pattern before you read the message. A single short reply means very little, because everyone has days where their phone sits face down. What you want to know is whether the gaps are getting longer, whether his messages are getting shorter across several days, and whether he has stopped sending anything he started himself. Those three trends together are the real signal. A man who likes you tends to send you things unprompted, a link, a photo of something he saw, a question about your day. A man who is pulling back stops doing that and only answers what lands in front of him. Also notice whether he still makes any plan, even a vague one. Interest in texting and interest in you usually show up together, so if he is still willing to talk about meeting, he is probably still in it, just less fired up than before.

When the conversation is thinning, the most useful replies are the ones that hand him a small, specific thing to respond to. A question with a clear answer works better than an open one, and a reference to something he said earlier shows you were paying attention without asking for anything. A light, funny observation about something in your own day can also re-open the thread without making him feel obligated to explain himself. The point is to make replying cheap for him. What to avoid is anything that asks him to justify the gap, reassure you, or commit to a plan he has not yet thought about. Those replies feel like pressure, and pressure is exactly what makes a man who is half checked out decide not to bother. Keep it short, keep it concrete, and let him come back on his own schedule.

Timing does a lot of the work here. If you send something at midnight when you are feeling the sting of his silence, the message tends to carry that feeling even if the words are fine, and it often gets a slow or no reply because it reads as an emotional bid rather than a conversation. Wait until your own day is settled and send when you would send to a friend. If he has gone quiet for a day or two, one message is reasonable. If he has not answered after that, do not send a second one for at least a few days, and do not send one that says you noticed. If he comes back with something warm or a plan, match his energy and move toward meeting. If he gives you a short answer or nothing, that is your answer, and the kind move for both of you is to let the thread end without a final message explaining it.