Tenant moved in adult child without my knowledge -

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My tenant's 12 month lease ends July 31 and I was informed they wish to renew. However I just learned the tenant moved in their 21 year old son without my knowledge and the son has been living there the past two or three months. The lease is specific that guests are not permitted to  stay more than seven consecutive days for any reason. The tenant has otherwise been good and pays rent regularly and on time so my aim is to try and find a positive outcome to this breach of rules by asking the son put in an application, be screened and added to the lease if he is planning to stay on for a while. I read a few responses to other similar posts that suggest ignoring these types of situations and letting them slide, but I'd like to keep this on the up and up and try to resolve it in an amicable manner. Given that the lease will be coming up for renewal in a few  weeks time, should my aim be to wait a while and add the son to the new lease or add him immediately to the current lease with an addendum? And should the parent and the son both be listed as "jointly and severally liable" even if the screening reveals the son is not in a position to assume this responsibility independently? Thank you.