No single act can "prove" love. Love is about choosing every day to respect and cherish a person. Gifts and acts of service are great, but they don't prove anything.
The main way to prove you love him is to be honest with him and treat him with respect. Show him you like being with him and there should be no doubt of your love. Treat him with kindness and fairness and he will feel the love you have for him.
Just make sure that your words match your actions. You don't need to buy your boyfriend tons of gifts or dinner to prove your love. Just try and help him out with things and be there for him.
