The chitosan is only soluble at pH < 6.5 - 7.0. So, when the medium is acid this biopolymer can be dissolved. Then, any acid can be used to dissolved the chitosan, as HCl or acetic
acid, I had already used both 1M. In addition, the HCl dissolves the chitosan easier than glacial acetic.
check this paper might be of help especially the introduction part on different strategies to prepare water-soluble chitosan. Article Acid-Treated Water-Soluble Chitosan Suitable for Microneedle...
I guess it depends on why he needs that chitosan for. According to the literature, the chitosan at high pH will crosslink itself and it slowly increases the solution viscosity, what would be nice for some applications. it also depends on chitosan Mw and %DD