Hi Isla,
Just wear what you are most comfortable in. It’s a casual meeting anyway, not a gala. If you feel most comfortable in sweatpants, a cute top and trainers, wear that.
Even though you have been chatting with him online for a month and you are just now getting to meet him for the first time, it doesn’t mean that you have to go out of your way to impress him with what you choose to wear.
The most important factor is that you are physically clean, your clothes aren’t wrinkled or dirty and you carry yourself well. That is really it. He is most likely not going to go all out either. Rest assured, he isn’t showing up in a tuxedo.
You are already used to wearing more casual clothing in your everyday life. Stick to this on your first date with this guy.
The last thing you want is to be constantly distracted or distressed by something that you are wearing that you only wore in order to impress him, but you were never really comfortable in to begin with. That leads to way too many awkward moments during the date as you try to adjust to the discomfort or having to make multiple trips to the bathroom, all leaving your date perplexed and uncomfortable.