He is right about the problem, but not about the strict liability. There is case law and statutory law in Massachusetts that creates a duty on the part of landowners to make safe any artificial conditions existing on their land that might attract and ultimately injure children, even trespassing children.
 
The theory behind the attractive nuisance doctrine is that children, because they are curious and inexperienced, will be drawn to these potentially dangerous conditions, but will not be able to appreciate the danger. The relevant statute is Massachusetts General Laws Chapter 231, Section 85Q:
 
Any person who maintains an artificial condition upon his own land shall be liable for physical harm to children trespassing thereon if (a) the place where the condition exists is one upon which the land owner knows or has reason to know that children are likely to trespass, (b) the condition is one of which the land owner knows or has reason to know and which he realizes or should realize will involve an unreasonable risk of death or serious bodily harm to such children, (c) the children because of their youth do not discover the condition or realize the risk involved in intermeddling with it or in coming within the area made dangerous by it, (d) the utility to the land owner of maintaining the condition and the burden of eliminating the danger are slight as compared with the risk to children involved, and (e) the land owner fails to exercise reasonable care to eliminate the danger or otherwise to protect the children.
 
However, note the language in part (e): "the land owner fails to exercise reasonable care." That language has been interpreted by Massachusetts courts as creating something less than strict liability. However, if a child injures himself on your wall (does not matter who built it, because you own it), it would certainly not be surprising if a fact finder determined that you were negligent and did not 'exercise reasonable care.' If I were you, I would get out there and take that wall down ASAP.