Ideal and the best for your baby is exclusive breastfeeding
till 6 months gets completed.
Usual we advice parents to start semi-solids after completion of 6 months.
Since you have already started weaning- no problem- you may continue with that.
Definitely supplementing with formula will reduce your breast milk output. If possible you may continue with breast milk only. And in between you may give the semi-solids you are giving. You may also start giving him water, fruit juices prepared at home.
If you feel, breast feeding will not be comfortable during travel, you may well start formula milk. Either of them can be used. But start them well in advance before your travel- if some intolerance or colic
(gas) problems happen- you can learn to manage them before your travel.. If it happens for the first time during travel, it will be difficult.
Its always better to avoid bottles- as it can result in many problems- like nipple confusion (he might refuse to breast feed after that, as feeding from a bottle will be easy), repeated ear infections, stomach infections, yeast infections
on the mouth etc..
So if you decide to give formula- best is to use tumbler and spoon.
Hope my answer helps you.
and wish your son good health.