You could write your own parser

namespace po = boost::program_options;

std::pair<std::string, std::string> short_flag_parser(const std::string& s)
{
    // one letter arguments work with single dash
    if (s[0] == '-' && s.size() == 2) {
        return make_pair(s.substr(1), std::string());
    } else {
        return make_pair(std::string(), std::string());
    }
};

int main() {
    desc.add_options()
        ("help, h", "produce help message");
    

    po::variables_map vm;
    po::store(po::command_line_parser(ac, av).options(desc).extra_parser(short_flag_parser).run(), vm);
    po::notify(vm);    

    if (vm.count("help")) {
        std::cout << desc;
        return;
    }
}