Among many reasons, the one told above contributes largely when he is deciding to live a lifetime with you. He is also afraid because he is unsure of the fact whether he'll be able to support both of you financially. You must be thinking you never forced him to bear all the expenses, then why? Well, that is what he thinks and never tells you, remember. You'll have to talk about this if you want to spend a lifetime with him.
The condition is known as Nocturnal Penile Tumescence (NPT) in which men's penis is hard in the morning too. It is because of the sexual fantasies that they dream of during the daytime. When nights are concerned, they tend to be at full liberty and also the hormones such as testosterone are higher in morning. The boner, however, can also be affected because of blood flow. Nonetheless, it doesn't matter whatever reason may be behind your boyfriend's boner in the morning; they'll never tell you about it.