Ok um, let's see..
I am gonna give you some advice on this matter as unbiased.

You do NOT necessarily have to be the so called romantic type and say or do mushy things every second.
You also do Not need to be serious and so "old fashioned" romantic lady with flowers and all for your sweetheart. No matter what, everyone needs to feel needed and personally I believe anyone and everyone has their own ways to make their sweetheart feel special.

You just have to figure out how.
Doesn't necessarily mean you have to say you love him and cook him dinner and all that, but just reciprocating the feelings in your own sweet ways even if its just a small kiss or anything, whatever you ARE comfortable with, just let him know that he is the one.
That's all you need to do.
How you do it is not the point, but if you do it, he will prolly notice it and know he is special to you, even if you always crack jokes and all.

That what a relationship needs, not flowers and cakes and kisses, but the feeling that is shared and exchanged between both individuals in their own ways that they know they are special to each other.
Every person is unique in their own ways, if you idea of romance is white water rafting and having lots of laugh, talk your sweetheart into it, make him see the world in a different sight, TOGETHER.
That's the keyword, and the key to everything.

All you need to make sure, that both of you know and make it clear that you are the one to each other, if that's how it is.
Everything will be ok once that part is settled.Besides, fun and jokes are something which even the gloomiest person gets a hang of soon enough, so don't worry, just talk to him openly about everything and your will see things will be ok.