SHOULD THIS BE A WORRY OR NOT?

It is reported that about 75% of the children on the autism spectrum have eating challenges and there is no shortage of literature, therapies and advice on what might help. 
As a parent with a child who is not eating a healthy diet you worry. As with any child who has eating issues preventing healthy diet you need to investigate the "why" even if the advice you get is "don't worry/they will be fine/it's a phase/they will grow out of it/they will eat enough so as to not be hungry...... etc, etc" 
With 30 years experience with autism and having a nutrition background I need to tell you that you do need to be concerned. Every child needs a healthy diet to promote healthy growth and to provide the nutrition they need to perform and achieve their personal best in play, school work, sports, music, art, sleep, behavior, social interaction, emotional growth, independence ~ LIFE! ~ all the things you hope for for your child. Autism itself may well play a major role and often alongside other issues. All need to be investigated.

If your child is not able to eat foods ~ chew, swallow, digest ~ start by making sure there is nothing physically wrong. Some children have needed surgical treatment that has been the solution to allow them to enjoy a healthy diet. 
Next, if you suspect there may be some food allergies ask your pediatrician for a referral for allergy testing to confirm or rule out your suspicions. 
If you know or suspect your child has sensory processing issues then they need to be addressed. I can recommend a good starting point http://www.sensory-processing-disorder.com/sensory-processing-disorder-checklist.html and consulting with an Occupational Therapist who specializes in SPD would be very beneficial.
You may now have to consider that although your child does not have allergies perhaps they have sensitivities or intolerances to certain foods. They may even be favorite foods! You can check this yourself by eliminating some foods to see if there are obvious benefits. This is an area where many parents of autistic children find some answers - specifically with gluten and casein.
I personally have experience with eliminating gluten from my diet and seeing significant health benefits in 3 weeks. It has been 3 months since I removed gluten from my diet and I won't be eating it anymore. It is certainly worth a try. I would say the same for casein, which will mean you eliminate all products made with cows milk.
This would appear to be a good resource to begin learning more about these dietary needs http://www.gfcfdiet.com/.
Whatever you find to be the problem, if there are still issues preventing your child from choosing to eat a wide variety of healthy foods then I would strongly recommend you investigate how The Eating Game can help. It will support any special diet you use as it is you, the parent, who presents the food choices for your child to use with The Eating Game. You can contact me at  jean@theeatinggame.ca if you want more information.
If you have anything to share that may help another person, please consider leaving a comment.