Hi and welcome. Hypotonia can appear with a range of different conditions. What you want is a proper diagnosis, not fluffy promises that all will be well.
His eating problems could well be caused by hypotonia, making it hard to work his jaws. I would have thought this could also affect his speech, but they will want to check him up, as hypotonia can also sometimes be associated with developmental delay and learning disabilities.
Both temper and exhaustion could be caused by the hypotonia: if it is exhausting just to keep his body together, he is going to feel exhausted and bad tempered, poor little soul.
My dcs both have a condition known as hypermobility syndrome. It's a connective tissue disorder. They have no learning difficulties, but have problems walking and using their joints.
Dd is now 12. She alternates between walking and using a wheelchair depending on whether she is having a good day or not. She can write, but needs to use a laptop for long projects or exams. Physio has done her a lot of good, but she also needs to pace herself so as not to overdo it. She had problems breastfeeding as a baby because of the hypotonia but was ok with solids. Looking at her last 5 years, it seems likely that she will always have a certain degree of disability and pain. But she is doing well at school and hoping to go to university eventually, so I think she'll still have a good life.
Ds is 8. He finds it difficult to write, and sometimes has pains walking, but is still active and able to do soccer club. He wears insoles and piedro boots, courtesy of the tax payers. We have no idea what his future holds. He could stay the same, get better or end up like dd in a wheelchair.
Some children with the condition grow completely out of all symptoms.
But this is only one condition; there are many. In fact, I am fairly certain that there are mumsnetters whose children have a combination of hypotonia and developmental delay, so someone may be around shortly to fill you in on that.
One thing I have learnt that I can tell you though: you will have to keep pushing for a diagnosis. And there will be doctors along the way who suggest that you are just being precious and over-worried. Go with your own feelings and your knowledge of your child.