Should The MRI Be Done Immediately For The Hurt Forehead?

My son took a tumble and hit his forhead on some cement, on Sunday around 1:30 pm although the impact was lessened by his hands being out. Later that evening when he was in bed he threw up. He woke up the next morning with some dry heaves and then whatever he ate soda cracker, gingerale etc within a few minutes would come back up. I figured it was the flu by Monday evening he ate dinner and 15 minutes later up it came. Tuesday we had the same thing. We tried giving him a powerade a Tablespoon at a time every 10 minutes and for 60 minutes he kept that down until I introduced a cracker then it al came back up. We started over and went 90 minutes and tried a bite of banana, he threw that up and then fell asleep for the night. Yesterday he begged me to eat and I gave him some chicken and he threw that up. SO we went back to the liquid every 10 minutes, Made it 90 minutes with a bite of banana after 60 minutes and another after 30 more minutes. I then took him to the doctors and he complained of getting a headache while ridding in the car. The doctor gave hom a perscription for Zoloft and the druggist gave us a generic substitute. He took it and 30 minutes later he ate dinner a sirloin burger, fries and a peach with ice water (because he was starving) he eventually threw up some clear liquid, after 30 minutes part of the burger came up. We went back to a Tbsp of powerade every 10 miutes and then switched that to water. He fell asleep for the night around 10:30 and woke at 9:30. He asked for breakfast, we gave him a pill and 30 minutes later two homemade cheese biscuits, they came back up within minutes. I waited for 20 miutes and gave hime 1/2 of a banana which he seemed to keep down and I took him to school. He is not weak, has no fever and seems to have energy he is just hungry! The doctor s office called and they have scheduled an MRI of his head for 7:45 am tomorrow. The problem is that he has a school field trip that travels about 60 minutes away and will be gone for most of the day. If he goes for the MRI in the morning he might miss the field trip, is it safe for me to postpone the MRI until tomorrow afternoon when he returns or am I just tempting fate?