In fact, there are several expressions, which I can roughly divide into three categories.
1. Oral expression means expressing love in person.
2. Use actions to express, which is a passive expression, such as suddenly holding her hand when only two people are together.
3. Subtle, that is, when we agree with each other, we express it with a little words and actions. For example, first tell him that he doesn't want to be single, miss a person, and after a while say that he can't find someone, he has to find her.
These are common and normal methods that can be used for reference.
But I suggest this method, because you two have a good relationship, but the relationship is not clear. If you express it directly and he accepts it, then there is no problem. If you don't accept it, your relationship may break up. So I don't think it's clear. You can express it tentatively, which will not only avoid embarrassment, but also weaken contradictions (meaning joining him and refusing will not hurt your feelings too obviously).
The method is as follows:
Tell him by SMS, email, or letter, for example, I need a girlfriend to accompany me to the movies (to travel, to the company party), and then give him movie tickets or something. If he comes, you can calmly hold the girl's hand. If not, what is it? You should be smart, too. Don't ask others why they don't come!
Pay attention to two points here. The time and place for watching movies or traveling must be convenient. Second, before sending a text message or letter, you can give her a romantic date to make her emotional, and the probability of success will be much higher.