As a Parent Educator, I can tell you that it is important to serve kids healthy foods, but, it is up to the children to eat them. That's what the doctor meant.
Last night, my daughter said she was starved. She wanted ice cream. i didn't have any. Plus, I knew she had eaten already. I put out a plate of organic, raw cauliflower w/ a homemade vinegar & olive oil dip. I put it out and walked away. She was coloring at the table. The plate was empty when I came back.
Try not to send out the message that you are expecting him to eat . He will feel the tension.
If he does have sensory issues, I would seek out an Occupational therapist.
Kids do go on food jags! This is very normal.
If he does NOT have sensory issues, this will probably be a phase.
Even as adults, we sometimes do this. I craved pasta all last wk! I made Tinkyada brown rice pasta every night for 6 days.
Oatmeal for breakfast is great. Try whole grain bread for his sandwiches.
I would buy jelly without corn syrup. If he wants grape juice, try squeezing a few grapes into it. Maybe you can smash a tiny bit of fresh, ripened banana into the pudding !