Some coeliacs have negative blood tests (but positive biopsies). Your DS's symptoms sound EXACTLY like my DS1's (he was diagnosed coeliac age 4). Pale floaty poos (steatorrhea), bloated, hard belly that fluctuates in size, stick thin legs, anaemic, lethargic
Telephone Coeliac UK /look at their website for help about getting diagnosed. Don't accept any delay. Here are the NICE guidelines about coeliac testing.
publications.nice.org.uk/coeliac-disease-cg86/guidance
If it was me, I would ask for another antibody blood test (as the coeliac may not have been advanced enough for antibodies to show up in the previous test, esp given your DS's young age) Check with Coeliac UK about exactly what tests should be done (10% of coeliacs are IGA negative and the ordinary coeliac blood tests don't work, so make sure the full range are done). I would also ask for a HLA blood test (which looks for the coeliac DQ2 and DQ8 genes - having them doesn't mean you are coeliac, but not having them rules it out). if the second antibody blood test was negative but he has a DQ2 or DQ8 gene then, I would ask to be referred to a paediatric gastroenterologist for biopsy anyway (we did this with DS2).
DON'T go gluten-free or reduce gluten yet - it will invalidate the testing. I think your DS will almost certainly improve on a gluten-free diet - but then it will be horrible/impossible to go back on to gluten for testing. If your son is coeliac, it is important to have the proper diagnosis - as it is a lifelong condition, and you won't have the full range of support without the diagnosis. Also, people without the proper diagnosis often don't follow the diet as strictly, and risk long term health problems as a result. Push to get all the tests / referrals done asap though. And finally, if the biopsy is negative, then I would go gluten-free, as strictly as for a coeliac (we did this with DS2 who had all the same symptoms but negative biopsy).
Both my DSs have been strictly gluten free for over a year now. DS1, who was in a similar condition to your DS at diagnosis, is a transformed child. Totally different shape (flat tummy, athletic rather than skinny build). Full of energy. Great appetite. Normal poos. Happy and not irritable any more (or only within normal 5yo bounds!) It has been wonderful, though also a big journey. It took us 2 years to get him diagnosed, and I wish we'd been quicker, but so glad we are now on the right path.
Wishing you all the best. Feel free to PM me if you need to.