There are three main points when you choose an eyeglass frame for your face shape:
First,it should be concordant with your personal features.
Second, the frame shape should contrast with your face shape.
Third, the size should be in scale with your face size.
Generally speaking, people with triangular face need a semi-fimless frame to accentuate the upper face and a top-heavy style to balance the width of the jaw. Try to avoid choosing low-set temples and narrow frames which will make you look quite strange.
For triangular face men, the round glasses are a good choice. The character of triangular face is wide in the place of mandible and ear. By wearing the round glasses with broad and clear edge, it will increase the width of frontal and eye brow part. In this way, I believe your boyfriend will look gentle.
There are many kinds of glasses suited for triangular face shaped men. Especially the big frame glasses are good, because the length of face is longer than its width. What you choose also depends on your skin color and coats. You can find what style suit for him. There are many different kinds of glasses in online stores.