If a condom is too tight or uncomfortable, he can lose his erection. It’s not you, it’s the condom, so don’t feel embarrassed.
A good way to avoid this problem is to try out different kinds of condoms. Lucky Bloke has a “Not Sure What Size to Buy” condom sampler if you think size could be the issue. They also offer lots of other samplers if you just want to explore your options. You can also get variety packs through a bunch of other online retailers like Amazon, Condom Jungle, Sustain, and Condomania.