Hello Georgi & welcome to the site.
Many marines have children & many live in married accommodation adjacent to the RM base, coming "home" 3 evenings out of 4 when they complete training & are not deployed.
Everyone gets 6 weeks leave per year plus 8 bank holidays.
On average he will get 2 weeks each at Xmas, Summer & Easter, but not always on specific dates.
After training, unless you are deployed away from your unit or on exercise, you can theoretically go home 3 weekend out of 4 when in the UK.
You can realistically expect to deploy for up to 6 months overseas, every 12 months on average in the current climate. Obviously different specialisations & trades may be required to deploy more often if there are manpower shortages.
If this is your first child as a couple, we would normally recommend he waits 6 months or more until after the birth to be sure that you can cope - hopefully in the company of supportive friends & family, before he begins training.
Good luck to all 3 of you!