Making a point to sit near him at Mass is a good suggestion. Trying to strike up a conversation with him afterwards is good, too. The more time you can spend with him, the more comfortable you both will be around each other.
To be honest, though, guys can be very clueless if the signs are too subtle (or maybe that was just me

). You might need to be more direct in order for him to really get it.
Not to say that it’s all up to you. I’m a big proponent that guys need to be bold and take the initiative, even if they are shy and hate rejection. And I say this as a guy who was painfully shy and really abhors rejection. I think a guy needs to get over those things and put himself out there if nothing more to show the girl that she is worth the risk.
But still don’t be surprised if he doesn’t pick up on it. Thinking back on my life, there were girls I was
very interested in who I convinced myself wanted nothing more than friendship. And yet I now recognize they were pretty clearly sending me signals that they reciprocated those feelings and I was too clueless to notice. Ah well, live and learn. I’m married now, so it all worked out despite my cluelessness. I learned my lesson and was bold when it counted.