What type of riding do you do? And what type of child seat do you have now?
My daughter is almost 5 now. We've had a Weehoo trailer for a year now and have used only a handful of times, for three primary reasons: (1) it's really long and we do most of our cycling in an urban environment where clearances are tight and I don't feel comfortable having my daughter so far back and so low down in traffic, plus it's really hard to park and secure; (2) the sucker is heavy and you can definitely feel the drag even with the child helping; and (3) it's a pain to transport and harder to put on and take off than a standard trail-a-bike.
We also had a Topeak Babyseat and that's what my daughter rode in most of the time before we got the Weehoo and for many of bike trips after we got the WeeHoo. My daughter is 44" tall and still fits in the Topeak Babyseat--she just recently outgrew it by weight and we sold it. She also fits in a Bobike Maxi seat just fine still and is within the 50lb weight limit for that.
We also had an Instep Pathfinder trail-a-bike that she started using from 4 years onward, but I wasn't fond of the "wiggle" from the seatpost mount. We recently got rid of the Pathfinder and bought a Burley Kazoo in the hopes of improving her pedal biking abilities. The Burley Kazoo is awesome. Even when she isn't pedaling, I barely feel it behind me; it's probably half the weight of the WeeHoo. We're keeping the WeeHoo around for a while to possibly use for my son (2.5), but I don't see my daughter using it anymore, unless we go for a really long--20 or 30 mile ride.
The times that we have used the WeeHoo are for a few long-distance, hilly trips where: (1) my daughter was likely to get tired, so the standard trail-a-bike wouldn't work; and (2) having her in the rear seat would be dangerous because my husband was popping wheelies trying to get up hills.
Unless you do a lot of distance riding or riding on protected bike paths, I would suggest:
(1) Finding a used child seat that will accommodate your son for 6 months or so until he's ready for a regular trail-a-bike; or
(2) Buying a regular trail-a-bike now and getting the Adams "backrest" with seatbelt; or
(3) Buy a cheap standard trailer for 6 months or so.
In hindsight, I regret buying the WeeHoo; it's very expensive for something that is useful for such a short period of time and I don't think it holds its resale value all that well (as compared to a Burley Piccolo or Kazoo).