Perhaps you can start by telling him now that you really want to enjoy everything that he has planned for you and that it would be so helpful if your "day of fun" could be broken into increments of 1 to 2 hours at a time. Ask him if his plans include outdoor activities because you will need to plan for that by using sunscreen, wearing a hat, sunglasses and appropriate protective clothing.
By breaking your day up into manageable increments, you will be much more able to enjoy all that he has planned. But let him know that you will not be able to do a full day of fun without having some rest periods in between. It sounds as if he really loves and cares for you, so I am sure that he will be understanding and accommodating!
In the meantime...let me wish you a very Happy Birthday
Peace and Blessings