Hello Banna ~ welcome
It is not uncommon to get to know one's colleagues very well, and to form a close bond with them.
If you are single, this can lead to romance.
However, if you are taken, then it can lead to one of two things ~ a platonic friendship or an affair.
It is very noble of him, given that he likes you, to respect the fact that you are in a relationship, and to keep the friendship platonic.
I'd say that if you want more than this, then it is for you to take the initiative, not him.
However, though it seems to suit some people, I certainly wouldn't recommend an affair. It can have a huge and negative effect on one's life.
If you are in love with this man, are you also in love with your partner?
If not, then is it right to let him think you still love him, when you don't ~ especially if you want a relationbship with this other man?
Are you being fair on him or yourself ~ or indeed your ex-colleague?
(Is it possible that your friend is gay??)