You are the project manager on this project. You need to plan, you need to allocate tasks to yourself and to your co-developer, and you need to manage the contact with your client. David Espina is absolutely right about that.
Every task takes time, and so you should plan accordingly. Client meetings, discussions with your colleague, and simply taking time out for planning and review are things that you should always allow for. Whether you have a documented plan or not (and I hope you do), you should not expect to spend 100% of your time on development work, otherwise you will never be on top of the job, and will always be playing "catch up". Far better to estimate that you need a number of hours each week for the PM work that needs to be done, and get that into your plan. If that means that your development will take longer overall than you had initially expected, you may have to make a tough call on how you handle that with your client. I strongly advise against compromising the PM part of the job, as that's the bit that will build and maintain the vital relationship with your client, and which will ultimately keep the client on-side and supportive.