Separation anxiety at almost 4 months??

My baby is starting to get really angry once he notices that I'm not around after about an hour. He'll only last that long with my husband and a little longer with my mom. I was not expecting him to do this at almost 4 months and I have to go back to work on the 17th. I have spent every minute of every day with him so I understand why but does anyone have any advice on how to help with it? My mom will be watching him once I go back for only 2 days out of the week and I don't want her stressing out about it.