Rickety is strange term to use. If the ladder did collapse, it may have been overloaded by his weight.
While most pull-down attic access ladders do have a weight limit, I can't recall there being a notice on the most attic doors advising users of the weight limit (like there are on all stand-alone ladders), but there might be.
Whether he elected to use the ladder even though he knew it wouldn't support his weight and it collapsed, or he just fell off and decided it was your fault, your first step is to get your insurance people involved to help defend against the claim or else pay it.
As to your credit, your history doesn't show claims by people on it, unless it becomes a court judgment lien against you. Your CLUE file will show the claim, however. The insurance industry keeps track of both persons and properties in CLUE and you can see if a claim has been filed. Ask your insurance agent about this.
Also, I am happy to see you know that an employee also has his own health insurance and his employer may be responsible for workman comp claims. Again, ask your insurance person about this aspect of your situation.