Why is my boyfriend losing his erection when we have sex and why doesn't he seem interested in sex anymore with me?

Before we can answer your question, we need more background information.

There are too many factors that can cause issues in the bedroom including age, health, stress, exhaustion, distractions, medications (including recreational drug use), etc. Any non-identifying information you can provide will help narrow down the list of possibilities.

One other question, have you tried talking to him about what is happening? I don't mean confronting him or anything along those lines. But calmly discussing the issues and what the causes might be.

As cat1864 said there is a great deal that could be causing this, you assume he doesn't want to sleep with you depending on background it could be exactly the opposite, if this is a new stage to your relationship it could even be nerves causing this problem. If you need more help you need to give more info.

Think he should see a doctor for Erectile Dysfunction before you get into a tizzy and think its all about you?