From being in a similar situation, I would suspect that your son is taking on an active parenting role while you are around to show that he is an active dad even though he goes out of town and probably feels guilty for not taking on any parenting or home responsibilities while he is gone. Who do you think takes care of this child when the father is away? Maybe you have alienated your daughter-in-law by being critical of her mothering or perhaps you monopolize the child and your son to the point of making her feel uncomfortable. This is a suggestion, the next time you are in their company, sit down on the couch and engage your son in a conversation to pre-occupy him and try your best to ignore your grandchild for a few minutes. Then, watch and see if your daughter-in-law takes on a more active role when the 2 of you are not being active. You will probably be surprised at the results. It takes a lot of work to care for an 18 month old on a day to day basis: providing 3 meals, changing numerous diapers, shopping and up-keep of baby items and foods, washing of extra clothes which are probably soiled, giving baths, transportation, providing entertainment of some sort, medical appointments, cleaning ears, clipping nails (very difficult at this age), picking up prescriptions, nursing a sick child back to health, dealing with toddler tantrums and all while your son that you describe as "doing everything" is "out of town". So, ask yourself if you think any of these needs are being neglected. Also, if your daughter-in-law is doing all of these things, then how much is your son or you really doing in comparison to this during the short time that you are around? I feel quite certain that if you were to ask the child's caregivers or doctors which parent does everything they might have a different answer for you.