It strikes me as unusual given that you don't know exactly what services you will be paying for. I always thought a solicitor was supposed to provide a writen bill of services.
There's also the risk that if you pay up front and he turns out to be useless and you want to change solicitor, will you get your money back?. at least if he hasn't been fully paid, he might be more incentivised to do his job properly
Without knowing the ins and outs of the case, it's hard to know if his fee is likely to be reasonable. However, if you're not comfortable, shop around