5yo tired with mild fever...

My 5yo son is on day 2 of cleanse and is very tired and developed a fever this evening. I'm assuming this is due to the cleanse, but needed some reassurance. How long could he feel bad....I'm sure this is different for everyone. My daughter and I did our first cleanse last August and made it through w/o any major side effects. Of course he is different and has his own set of issues (allergies, enlarge tonsils/adenoids, eczema, asthma when younger....) I'm hoping this cleanse will help him overcome these issues.....

While I realize everyone is different, how long should I let him go feeling tired and feverish?

Thanks for any advice..