Hi Lea - jenna had this recently when she got a nasty dose of gastro. Every dr I told about it said it wasn't really a concern.
Not unless it hangs around for more than 4-5 days or so.
Keep him hydrated as the mucous in Jenna's poo appeared a day or two before she started getting really ill, and wouldnt drink anything. Luckily I tried to keep her hydrated as much as possible before the bad patch hit so she had some stores.
I wouldn't bother doing anything about it unless it gets worse, gets bloody too, or hangs around for more than 3-4 days.
Hope he gets better soon!