The next obvious step is to call him, right?
Here is what you can say:
"Hi, this is ... I hope you don't mind me calling you. I got your number from ... . I had a good time the other day.
Simply wanted to tell you.
Look, there is a party coming up next Saturday and I was planning on going there with some friends.
If you want to show up, I'm sure it will be fun..."
This is a kind invitation.
If he can't or has other plans, no worries.
The first step is to give him a clear message that you are interested.
Keep it light and make him realize that you will have fun whether he is there or not.
You are simply opening a door for him.
Now, when you make such call, keep in mind that it's 50/50.
He could respond or not.
This means that you want to have a strategy in case he does not go with it.
Change the topic.
Ask him about his day and what he enjoy doing usually in the week ends, etc.
Keep it light and keep it short (5 min max).
Finish on a high no matter the outcome.
Say something like: "Well, it was nice catching up with you. Hope you can make it on Saturday... It will be fun"
It is very important not to bring any questions, issues or insecurities on your side.
A one time call like this one is exciting if you keep it short and exciting.
You need to feel energized after it.
After that, move on!
The ball is on his side.
You initiated the connection.
Now, let it go and keep your options open.
Don't expect anything.
Simply be proud of yourself for having taken the step!
Any action is always a victory in the dating scene, no matter the outcome.
Go for it!