If you feel that your boyfriend is putting his grown child before you in your relationship, it's important to have an open and honest conversation with him about how his behavior makes you feel. 

It's important to express your feelings in a calm and respectful manner and to listen to his perspective as well. He may not be aware of how his behavior is impacting your relationship, and hearing your perspective may help him to understand.

During the conversation, you can also set boundaries in the relationship. For example, you can let your boyfriend know that you need more time and attention from him, and that you expect him to prioritize your relationship. 

It's important to be clear about what you need and to communicate your expectations in a direct and assertive way.

If your boyfriend does not respond positively to the conversation and continues to prioritize his grown child over your relationship, it may be necessary to seek the help of a relationship counselor or therapist. 

A professional can help you and your boyfriend to work through any underlying issues and to develop a healthier and more balanced relationship.

It's also important to remember that you deserve to be with someone who values and respects you and your relationship. If your boyfriend is unable to prioritize your relationship, it may be necessary to consider other option.