Assuming your Bermuda tests safe I would get him off the straw. Straw has very little nutrition compared to hay which is probably why he is so thin. It's even worse at this young age. Straw is all the rage for feeding donkeys these days but a very bad idea in my opinion https://drkhorsesense.wordpress.com/2020/11/16/the-facts-about-feeding-straw/ . Healthy donks are muscular and even shining https://www.bensonranch.com/wp-content/uploads/2014/11/Silverado-hess-side-cf-600.jpg . You won't see this with straw fed donkeys.

The straw feeding recommendation is fostered by the Donkey Sanctuary in the UK. To be fair, it can be more difficult to find hay or haylage that meets the requirements for low sugar and starch. Some of their ryegrasses  may even reach dangerous fructan levels. None of that is an issue here.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001