My son is two years old and had a severe constipation from last two days?

See pediatrician. : You can try something like miralax (polyethylene glycol) and consider fiber supplements once it resolves to avoid future events. See your pediatrician.

Short or long term?: If this is just a one time thing for a few days, usually just some extra fluids and/or juice. You could do a one-time infant glycerin suppository as well (you know where that goes!). If it is a long term problem, then some diet changes with less milk products and more fiber needed. In addition, a product called MiraLAX (generic peg or polyethylene glycol) used on a routine basis is helpful.