Hi and welcome to the forum.

It would help to know if the Synapse is also Sora, and what road bike your boyfriend was riding before (to understand better his tastes). Also the price difference and your budget.

I think 58 is the right size for him, I'm 6'3'' (192 cm) and ride 58 (the biggest frame size available in Japan where I live) although by textbook standards it's probably a tad small.

I can definitely recommend the CAAD 8 - for the price it's a great bike. Although I've upgraded to Supersix EVO 105 last year (with zero regrets), I still ride the CAAD every now and then and feel great on it. It's also the one that got me hooked on cycling.

I know of people that are enthusiastic about Synapse (unfortunately I have never tried it myself) but I wouldn't worry too much about the "comfier" aspect unless he's really going to get into endurance and do, say, 100km+ rides.