I just started thinking about my ex boyfriend again and I want to text him something to let him know I have been thinking about him and I don't know what to send him so, any suggestions?

If you must send something in general like " Hi, was just wondering how youre doing?"  and leave it at that as that requires a non committal answer and is only being polite and then see what happens. If no reply in a few years, hes lost interest in you and you better move on.  Just kidding, only give him maybe 48 hours and then move on.  Good luck