I would get some blood work done before just adding supplements. Its cheap enough and if there is a problem you can get that addressed. A few things, some horses are lazy by nature. Whats his age, if he is young and under a heavy workload or if hes a pleasure horse being asked to shift his body weight to an unnatural position would should up as straining and lack of energy. Also if he is on any Gamma oryzanol products they can impact work effort and any muscle diseases such as HYPP or Azotoria there are many stress induced deficiences.